Cell Cycle and Mitosis

Cell and Nuclear Division

DNA packaging

• In eukaryotic cells, DNA is contained in the nucleus

• Each cell contains 2 meters of DNA within the nucleus

• DNA is “packaged” to fit all of the DNA within the nucleus of each cell

Nucleosomes

• A nucleosome is a stretch of DNA that is wrapped around a core of 8 histone proteins

  • Two copies each histone protein: H2A, H2B, H3, and H4

• A 9th histone protein (H1) holds the DNA around the nucleosome

• Nucleosomes are connected with Linker DNA - a stretch of DNA wrapped around histones that connect neighboring nucleosomes

• This is often referred to as “beads on a string”

Chromatin and Chromosome

• The DNA is further packaged into chromatin - loosely packed DNA found in non-dividing cells

• When a cell is actively dividing, the chromatin condenses into chromosome - tightly wound DNA found in actively dividing cells

Chromosome Structure

• After DNA replication occurs, the 2 DNA molecules (chromatids) are tethered to each other at the centromere

• When the DNA packs into chromosomes, we see the x-shaped structure of a duplicated chromosome (replicated chromosome)

Chromatids are sister chromatids if they are connected at the centromere

DNA packaging and Gene Expression

• Tightly packaged DNA (chromosomes) makes the genes inaccessible

  • No transcription

  • No gene expression

• One way to control gene expression is to manipulate the level of DNA packing; looser packaging = more gene expression

Unicellular Organisms

Unicellular organisms:

• Cell division = reproduction because a new organism is produced

• It is a type of asexual reproduction because new cells are genetically identical to each other and the original cell

Cell proliferation in multicellular organisms

• Growth

  • Increase the size and complexity of multicellular organisms by producing more cells

• Cell replacement

  • Tissues such as skin cells require routine replacement

  • Helps to maintain healthy tissues and replace dead cells that are lost

  

• Tissue repair

  • Wound healing requires cells to proliferate to repair the damaged tissues

Cell proliferation ensures continuity of genetic information across all cells within an organism

The cell cycle

• The sequence of events that a cell undergoes, including growth, DNA replication, and nuclear and cytoplasmic division.

• Two parts:

  • Interphase

  • Miotic phase (M phase)

Interphase

Interphase is composed of 3 subphases:

• G₁ - gap 1 - Involves cell growth and normal metabolic functions

• S - Synthesis - DNA replication occurs

• G₂ - gap 2 - involves cell growth and preparation for nuclear division

Interphase: G₁

• the cell approximately doubles in size

• Metabolic processes occur such as protein synthesis

• Mitochondria (and chloroplasts in plants) divide using binary fission

Interphase: S

• DNA is replicated within the nucleus

• Remember: DNA replication is semi-conservative

Interphase: G₂

• Continued growth

• Synthesizing microtubules (proteins0 and other proteins necessary for cell division

Mitotic Phase (M Phase)

The mitotic phase (M phase) is composed of 2 subphases:

• Mitosis - the division of the nucleus

• Cytokinesis - the division of the cytoplasm

Cytokinesis happens simultaneously with the end of mitosis

Mitosis

• The division of the nucleus

• At the end of mitosis and cytokinesis, 2 genetically identical daughter cells have been produced

  • Genetically identical to each other

  • Genetically identical to the original parent cell

Stages of mitosis

Mitosis is divided into 4 stages:

• Prophase

• Metaphase

• Anaphase

• Telophase

Prophase

• Chromatin condenses into chromosomes - remember, they are duplicated chromosomes because they have just been replicated during the S phase

• The nuclear membrane breaks down

• Spindle fibers form

• Spindle fibers are made of microtubules and are responsible for the movement of chromosomes during mitosis

• Sometimes they are referred to as “the mitotic spindle”

• Plant cells use microtubule organizing centers (MTOCs) to organize the spindle

• Animal cells use centrosomes (a type of MTOC that contains centrioles)

• During prophase, the MTOCs migrate to the poles of the cell

Metaphase

• Duplicated chromosomes line up along the metaphase plate at the equator of the cell

• spindle fibers attach to the chromosomes at the kinetochore - motor proteins that are located in the centromere region

Anaphase

• Centromere splits and sister chromatids separate and move away from each other towards the poles of the cell

• Spindle fibers are responsible for chromosome movement

Once the sister chromatids have separated, they are considered 2 unduplicated chromosomes

Telophase

• Chromosomes decondense into chromatin

• The nuclear membrane reforms around the two new nuclei

• Spindle fibers

Cytokinesis

• The division of the cytoplasm

• Separates the parent cell into 2 daughter cells

• Sometimes simultaneous with telophase

• Proceeds differently in plant cells v. animal cells

Animal cell cytokinesis

• Actin and myosin proteins form a contractile ring at the center of the cell

• The contractile ring pinches the cell membrane in and forms a cleavage furrow

• The furrow deepens so it eventually splits the cell into 2

Plant cell cytokinesis

• Vesicles carrying cell wall material assemble into the cell plate

• The cell plate grows outwards and will eventually form the cell wall between te 2 daughter cells

Binary Fission

• Type of cell division performed by prokaryotes (and mitochondria & chloroplasts)

• Steps:

  • 1. DNA replication

  • 2. Cell elongation

  • 3. Cytoplasm divides

  • 4. Daughter cells produced

Mitosis and Binary fission produce genetically identical cells - in general, they do NOT increase genetic diversity… except for mutations during DNA replication

Cell cycle control, cancer, and the mitotic index

The cell cycle must be tightly regulated to maintain healthy tissues

Cell cycle control

• The cell cycle contains a series of checkpoints that help to regulate the cell cycle

  • G₁ checkpoint

  • G₂ checkpoint

  • M checkpoint

• Internal and external controls are molecules that act as stop-and-go signals at the different checkpoints

G₁ Checkpoint

• The G₁ checkpoint determines if the cell will eventually go on through the rest of the cell cycle

• A “go” signal at the G₁ checkpoint will allow the cell to continue through the rest of the cell cycle

• A “stop” signal at the G₁ checkpoint will cause the cell to exit the cell cycle and enter G₀ - a phase where the cell is not preparing to divide

• Some cells can re-enter the cell cycle from G₀

• Other cells are in “terminal G₀” and cannot re-enter the cell cycle

G₂ Checkpoint

The G₂ checkpoint checks for:

• Has the cell grown enough?

• Has the DNA replicated fully?

• Has the cell produced enough energy, proteins, organelles, etc. in preparation for cell division?

A “go” signal at the G₂ checkpoint allows the cell to enter the mitotic phase

M checkpoint

• The M checkpoint occurs during the metaphase of mitosis

• Check to make sure all of the chromosomes have:

  • Attached to spindle fibers

  • Lined up at the metaphase plate

• A “go” signal at the M checkpoint allows the cell to enter anaphase

External regulators

• response to events outside the cell, often directing cells to speed up or slow down the cell cycle

• Examples;

  • Growth factors

  • Anchorage Dependence

  • Density-dependent inhibition

Growth factors

• an important group of external regulatory proteins

• Stimulate the growth and division of cells

• Important during embryo development and wound healing

• Example:

  • PDGF (platelet-derived growth factor) stimulates the division of human fibroblast cells in culture

Anchorage dependence and Density-dependent inhibition

• most animal cells exhibit anchorage dependence, in which they must be attached to a substratum to divide

• Density-dependent inhibition:

  • Crowded cells stop dividing

  • Prevents excess cell division

Internal regulators

• Response to events inside a cell and allow the cell cycle to proceed only when certain events have occurred

• Example:

  • Cyclins and Cyclin-dependent kinases (CDKs)

Cyclins and CDKs

• Cyclins are a family of proteins that regulate the cell cycle by activating cyclin-dependent kinases

• Cyclin-dependent kinases (CDKs) are enzymes that, when activated phosphorylate proteins to progress the cell cycle

• CDKs are always present, but not always active

• Cyclins activate CDKs by binding to them creating a cyclin-CDK complex

• Different cyclins accumulate during different stages of the cell cycle, thus activating different CDKs at different times

• Once the cyclin-CDK complex has completed its task, the cyclin is degraded and the CDK is deactivated

The cyclin must reach a critical concentration for the cell to progress to the next stage

Fluctuations of cyclin concentrations control the cell cycle

Cancer

Cell cycle control genes

• the proteins that act as regulators for the cell cycle are coded for by genes

• When those genes are mutated, that can cause a breakdown in the cell cycle control system

• Mutations are any change to the DNA sequence

Proto-Oncogenes

• Proto-oncogenes are genes that code for proteins that help promote cell growth and division

• Mutations to proto-oncogenes can lead to the proteins being overexpressed, leading to uncontrolled cell division

• When a proto-oncogene becomes mutated, it is called an oncogene

Tumor suppression genes

• Tumor suppression genes are genes that code for proteins that normally slow down or prevent cell division

• They can also trigger apoptosis - programmed cell death

• Mutations in tumor suppressor genes will lead to malfunctioning proteins - the absence of the protective function of these proteins leads to uncontrolled cell division

Benign Tumors

• Uncontrolled cell division can lead to the accumulation of an abnormal mass of cells - tumor

• Benign tumors - abnormal growth of cells that are not cancerous

  • Grow slowly, well-defined margins, don’t metastasize (spread to other parts of the body)

  • Can still cause issues depending on location and size

Malignant tumors

• Malignant tumors - cancerous, growing and dividing more rapidly than benign tumors

• Lack well defined borders

• Undergo metastasis

  • Primary tumor - original tumor

  • Secondary tumor - a tumor that forms in a new location

Treatments for tumors

• Benign tumors are often treated with surgery

• Malignant tumors are often treated with a combination of:

  • Surgery

  • Radiation

  • Chemotherapy

Mitotic index

Mitotic index

• Measure of the proportion of actively dividing cells in a population

• Value of 0 to 1, or it can be written as a percentage (0% - 100%)

• The more cell division, the larger the mitotic index

• A high mitotic index is NOT always an indicator of a cancerous tumor

• Some cells/tissues/developmental stages will have high rates of cell division

  • Embryonic development

  • Epithelial cells (skin & lining of the digestive tract)

  • Meristem cells in plants (areas of growth in roots/shoots)

Mitotic index can be used as a diagnostic tool for cancerous tumors - must be compared to the MI of healthy tissues

Mitotic Index

Meiosis

Purpose of reproduction

• Reproduction is one of the processes of life

• The way by which organisms pass on their genes to future generations

• Ensure the continuity of their species

• Also necessary for natural selection to occur

Asexual reproduction

• The production of genetically identical offspring from a single source of genetic information

• It occurs in prokaryotes, fungi, many plants, and some animals

• Binary fission - a type of cell division performed by prokaryotes

• Mitosis & cytokinesis - asexual reproduction performed by unicellular eukaryotes

• Budding - a new organism develops as an outgrowth (bud) from the parent organism, eventually detaches and becomes an independent organism

• Fragmentation - The parent organism breaks into fragments and each fragment develops into a new organism

Asexual reproduction - Advantages

• A large number of offspring is produced in a short amount of time

  • An advantage in a stable environment if the parent organism is well adapted to the environment

• Less costly in; time, energy, and resources

• Less complex process

Asexual reproduction - disadvantages

• Does not increase genetic variation/diversity

• No genetic variation means that if the environment changes, the species could be completely wiped out

• Harmful mutations have a widespread effect

Sexual reproduction

• The production of genetically different offspring from two sources of genetic information

• The offspring inherit some genetic information from each source (parent)

• Occurs in multicellular plants and animals

Sexual reproduction - advantages

• Increases genetic variation/diversity - offspring are similar but not identical to each other and the parents

  • Advantages in changing environments - more genetic variation increases the chance of survival of the species

Sexual reproduction - disadvantages

• In general, fewer organisms produced in a longer period of time

• More costly in; time, energy, and resources

• More complex process

• Harder to accomplish because gametes must fuse

Process of sexual reproduction

1. Production of gametes - using meiosis

2. Fertilization - fusion of haploid gametes to create a diploid zygote

3. Development of offspring-cell proliferation

Sources of genetic variation

In sexual reproduction there are 4 sources of genetic variation:

• Crossing over during meiosis 1

• Independent assortment of homologous chromosomes during meiosis 1

• Random orientation of homologous chromosomes during meiosis 1 and sister chromatids during meiosis 2

• Random fertilization during the fusion of gametes

Meiosis

• A process of cell division that produces 4 haploid gametes that are genetically unique

• Meiosis is considered a reduction division because it takes 1 diploid cell and creates 4 haploid cells by separating the homologous chromosomes

• Gametes are cells used in sexual reproduction

Review: Chromosome number

• Diploid cells contain 2 copies of each chromosome -2n

  • Ex. Humans 2n = 46

  • Body cells are somatic cells and they are diploid

• Haploid cells contain 1 copy of each chromosome - n

  • Ex. Humans n = 23

  • Gametes (sex cells - sperm and eggs) are haploid

• A duplicated chromosome is still only 1 chromosome

• This means that a diploid cell is still diploid before and after S phase (synthesis)

• DNA replication does not increase the number of chromosomes, it increases the number of chromatids

Review: Homologous chromosomes

• Diploid cells have pairs of chromosomes

• The chromosomes within the pair are homologous chromosomes

• Homologous chromosomes:

  • are the same length

  • have the same gene loci in the same order and location

  • have the centromere region in the same location

• In humans, the pairs of autosomes (chromosomes 1-22) are homologous

Before Meiosis

• DNA replication must occur before meiosis can proceed

• At the beginning of meiosis 1, the parent cell is a diploid cell with duplicated chromosomes (2 sister chromatids in each)

Meiotic Divisions

• Meiosis consists of 2 rounds of division: Meiosis1 and Meiosis 2

• Each round of division progresses through prophase, metaphase, anaphase, telopase, and cytokinesis

Meiosis 1

Meiosis 1: Prophase 1

• The nuclear membrane breaks down

• Spindle fibers start to form

• Microtubule organizing centers (MTOCs) move away from each other toward opposite poles of the cell

• Chromatin condenses into (duplicated) chromosomes

• Homologous chromosomes pair up during synapsis creating tetrads/bivalents

Tetrads remain paired up until anaphase 1

• Crossing over occurs when non-sister chromatids exchange equivalent segments of DNA

• The exchange of DNA happens at the chiasmata - the x-shaped regions where crossing over has occurred

• Crossing over creates recombinant chromatids

Meiosis 1: Metaphase 1

• Tetrads line up along the metaphase plate

• Independent assortment of tetrads - each tetrad pair lines up independently of the other pairs - there is a random orientation towards the poles

• Spindle fibers attach to the kinetochores on either side of the homologous pairs

Meiosis 1: Anaphase 1

• Homologous chromosomes separate from each other

• Sister chromatids remain connected at the centromere during Anaphase 1

• Spindle fibers are responsible for moving the homologous chromosomes away from each other

Meiosis 1: Telophase 1/cytokinesis 1

• Homologous chromosomes have reached the poles of the cell

• Chromosomes decondense into chromatin

• Nuclear membrane reforms around the 2 new (haploid) nuclei

• Spindle fibers break down

• Cytokinesis occurs by producing 2 daughter cells that are:

  • Haploid with duplicated chromosomes

  • Non-identical (genetically unique)

• After Meiosis 1 is complete interkinesis occurs - a period of rest, no DNA replication occurs

Meiosis 2

Meiosis 2: Prophase 2

Meiosis 2 is very similar to mitosis, each daughter cell produced after Meiosis 1 will proceed through Meiosis 2

• Chromatin recondenses into chromosomes

• The nuclear membrane breaks down

• MTOCs migrate to opposite poles of the cell

• Spindle fibers begin to form

Meiosis 2: Metaphase 2

• Spindle fibers attach to the kinetochores at the centromeres of the duplicated chromosomes

• Duplicated chromosomes (sister chromatids) line up along the metaphase plate

• Sister chromatids show random orientation toward the poles - remember they are no longer identical

Meiosis 2: Anaphase 2

• Centromeres split

• Sister chromatids separate from each other and move towards the opposite poles of the cell

• Spindle fibers are responsible for the movement of chromosomes

Remember. once the sister chromatids are separated, they are called unduplicated chromosomes

Meiosis 2: Telophase 2/Cytokinesis 2

• Chromosomes reach the opposite poles of the cell

• Chromosomes decondense into chromatin

• Nuclear membranes form around the 2 (haploid) nuclei

• Spindle fibers break down

• Cytokinesis occurs by producing a total of 4 daughter cells that are:

  • Haploid gametes with unduplicated chromosomes

  • Non-identical (genetically unique)

• These daughter cells will mature into the gametes

Review

Review: Karyograms

• A karyogram shows an image of an organism’s chromosomes

• In humans:

  • Chromosomes 1-22 are autosomes and each pair is homologous to each other

  • Chromosome 23 are the sex chromosomes and are not homologous

Aneuploidy

• The presence of an abnormal number of chromosomes in a cell

• Trisomy - 3 copies of a chromosome

• Monosomy - 1 copy of a chromosome

• The most common type of aneuploidy compatible with life is trisomy 21 which causes Down syndrome

Nondisjunction

• An aneuploidy is created if nondisjunction occurs

• Nondisjunction is a failure of homologous chromosomes to separate during meiosis 1 or a failure of sister chromatids to separate during meiosis 2

• Creates gametes with a missing chromosome or an extra chromosome

• If that gamete is involved in fertilization, then the zygote (and therefore the offspring) will have a monosomy or trisomy

Nondisjunction during mitosis does not cause aneuploidy in offspring

Karyotyping

• A karyotype is created when analyzing a karyogram - it can determine if there is an aneuploidy

• When a woman is pregnant, a karyotype can be done on the fetus by collecting fetal cells using:

  • Non-invasive prenatal testing (NPT)

    • Blood draw

  • Aminocentesis - the collections of amniotic fluid

  • Chorionic Villus Sampling - the collection of placental tissue

Mendelian Genetics

Gregor Mendel

• Heredity is the passing on of characteristics/traits from parents to offspring

• Genetics is the field of biology that deals with heredity

• He investigated 7 pairs of traits, performed crosses, and observed the offspring

Model Organisms: Pea Plants

• Mendel had easy access to pea plants because of his monastery

• They were a good model organism because:

  • They can reproduce sexually

  • They can self pollinate and Mendel could control the cross-pollination between plants

  • They have a relatively short reproductive cycle and produce many offspring

Mendelian Traits of Pea Plants

Each trait had 2 different alleles - different versions of the same gene

• Flower color (purple or white)

• Seed color (yellow or green)

• Sed shape (round or wrinkled)

• Pod color (green or yellow)

• Pod shape (smooth or bumpy)

• Flower position (mid-stem or end of stem)

• Plant height (tall or short)

Alleles

• Alleles are different versions of the same gene

• Alleles are created by mutations to a gene sequence

• Many alleles are due to single nucleotide polymorphisms (SNPs)

Mendel’s Peas

P generation

• Mendel created true-breeding plants - a line of plants that only produced plants with a specific trait when allowed to self-pollinate

• Mendel’s P generation consisted of 2 true-breeding plants of opposite traits (ex. a true-breeding purple flower plant and a true-breeding white flower plant)

F1 Generation

• Mendel performed the first cross with the P generation where he cross-pollinated the true-breeding plants

• The resulting offspring were called the F1 generation

• He called these the hybrids (monohybrids)

• 100% of the hybrids exhibited only 1 of the two versions of each trait (ex. purple flowers crossed with white flowers produced 100% purple flowers)

F2 Generation

• Mendel allowed the hybrids (F1 generation) to self-pollinate to produce the F2 generation

• The offspring in the F2 generation exhibited both versions of the traits in predictable ratios (ex. ¾ purple flowers and ¼ white flowers - 3:1 ratio)

1st concept of Mendelian genetics

2 alleles

• Traits that exhibit the patterns of Mendelian genetics only have 2 alleles

• Within each organism, there are 2 copies of each gene, one on each chromosome in a homologous pair (at the same loci)

• Those copies could both be the same, or they could be different

2nd concept of Mendelian genetics

Biparental Inheritance

• For each trait, an organism inherits 2 alleles, 1 from each parent/gamete

• Remember - these alleles could both be the same or they could be different

Autosomal traits

• Traits that exhibit Mendelian genetics have genes that are found on the autosomes (chromosomes 1-22)

• this allows for them to be inherited in a biparental pattern

• Traits determined by genes that are found on the sex chromosomes (X or Y) are not considered to be “Mendelian”

3rd concept of Mendelian genetics

Dominance vs. Recessiveness

• of the 2 different alleles for any trait, one is dominant and one is recessive

• When an organism has 2 different alleles, the dominant allele will be the one that is expressed

• The recessive allele is only expressed in the absence of the dominant allele - aka when an organism has two copies of the recessive allele

4th concept of Mendelian genetics

Law of segregation

• In the parent, the alleles of each trait segregate (or separate) into each gamete so each gamete only receives one copy of each gene

• This is because the homologous chromosomes separate during Meiosis 1

Segregation of Alleles and Biparental Inheritance

• These 2 concepts are tied directly to the process of sexual reproduction: Gamete formation and fertilization

• During gamete formation, the homologous chromosomes separate during Meiosis 1- (segregation of alleles) - because gametes are haploid, each gamete to only contribute 1 copy of each gene to their offspring (biparental inheritance)

• During fertilization, the fusion of sperm and egg creates a diploid zygote

• The fertilization process provides the offspring with 1 copy of each gene from each parent/gamete (biparental inheritance)

• Tied directly to the chromosome numbers:

  • Before Meiosis: Diploid cell

  • After Meiosis: Haploid gametes

  • After fertilization: Diploid zygote

Writing Alleles - Autosomal traits

• The alleles for an autosomal trait are given letters

  • Capital for the dominant allele

  • Lowercase for the recessive allele

Genotype - Autosomal Traits

• The combination of alleles is the genotype of an individual

• Genotypes for 1 trait are written with 2 letters to represent the 2 alleles on the homologous chromosomes

• Possible combinations: FF, Ff, and ff

• Organisms are homozygous if they have 2 copies of the same allele

  • FF = homozygous dominant

  • ff = homozygous recessive i

• Organisms are heterozygous if they have 1 copy of each allele"

  • Ff = heterozygous

Genotype and Phenotype

• the genotype determines the phenotype (with a few exceptions)

• The phenotype of an individual is the expression of the genes (ex. physical appearance or metabolic characteristics, etc.)

• Homozygous dominant and heterozygous genotypes will result in the dominant phenotype

• Homozygous recessive genotypes will result in the recessive phenotype

Recessive Disorders

• Many genetic disorders are inherited in an autosomal recessive pattern - the gene’s locus is on an autosome, and the allele is recessive, so an individual must be homozygous recessive to express the recessive disorder

• If someone is heterozygous for a recessive disorder, they are said to be a “carrier”

• Carriers have the normal (dominant) phenotype but have 1 copy of the recessive allele that they can pass on to their offspring

• Many partners will get genetically tested to see if they are carriers of the same trait

• Example: Phenylketonuria or PKU

Phenylketonuria (PKU)

• PKU is caused by a mutation in a gene on chromosome 12 (autosomal)

• This gene codes for an enzyme called phenylalanine hydroxylase (PAH)

• PAH converts the amino acid phenylalanine (Phe) into tyrosine (Tyr)

• The mutation creates a malfunctioning enzyme

• Individuals that are homozygous recessive are unable to break down phenylalanine (Phe)

• Phenylalanine builds up to toxic levels and can cause: a musty odor from the skin and urine, fair skin, eczema, seizures, tremors, and hyperactivity

• If the condition is left untreated, brain damage can occur

• PKU can be managed with a tightly controlled, low-protein diet

5th concept of Mendelian genetics

Law of independent assortment

• Each pair of alleles segregates independently of each other pair of alleles during gamete formation

• AKA the way each tetrad lines up during metaphase 1 has zero impact on the other tetrads

• Assumption: the gene loci are on different chromosomes

Law of independent assortment

• If an organism is heterozygous for 2 traits, their genotype would be AaBb

• Remember: Each gamete only gets 1 copy of each gene (aka 1 of each type of letter)

When we perform a 2 trait cross, we assume that the traits are independent (and therefore Mendelian)!

Pedigrees

Pedigree charts

• Illustrate the inheritance of a trait through a family’s history

• Useful for illustrating genetic disorders and to help predict the probability of future generations having the disorder

Pedigree Symbols

• Circles are females

• Squares are males

• Filled in shape = individual has the trait

• Unfilled in shape = individual without the trait

• Horizontal line between a circle and square = mating lines (children branch off of the mating lines)

Identifying individuals

• Generations are labeled with Roman numerals starting at the top row

• Individuals are labeled with Arabic numerals moving left to right

Types of Inheritance

• Autosomal dominant

  • AA, Aa = has trait

  • aa = doesn’t have the trait

• Autosomal recessive

  • AA, Aa = doesn’t have the trait

  • aa = has the trait

• Sex-linked recessive

  • X^AX^A = female without trait

  • X^AX^a = female without trait (carrier)

  • X^aX^a = female with trait

  • X^AY = male without trait

  • X^aY = male with trait

Determining Inheritance

Autosomal Dominant

• Means filled in shapes are either AA or Aa

• If a child has the trait, at least one parent must also have the trait

• If both parents have the trait, and at least one of the children doesn’t have the trait, it must be autosomal dominant

• Means filled in shapes are either AA or Aa

• If neither of the parents has the trait, the offspring will not exhibit it either

• If both parents do have the trait, but the offspring does not, then the child will be homozygous recessive and the parents will be heterozygous or carriers

Autosomal recessive

• This means filled-in shapes are aa

• If both parents are affected (filled in), the offspring should be also

• If the parents are both unaffected, but the offspring has the trait, then the parents must be heterozygous

X-linked recessive

• Means filled-in shapes are X^aX^a (female) and X^aY (male)

• Mostly males with the trait suggest X-linked, but sex linkage cannot be confirmed

• A female with the trait must have a father with the trait and 100% of her sons would also have the trait

• An unaffected mother can have affected sons if she is a carrier (heterozygous