Honors Biology Unit 6: Meiosis and Genetics

 A chart of homologous chromosome pairs used to pinpoint unusual chromosome numbers in cells.  (Humans = 23 Homologous Pairs / 46 total chromosomes)

Karyotype

refers to how genetic traits are passed down through the 22 pairs of non-sex chromosomes (autosomes). It is a fundamental concept that explains the probability of a child inheriting a particular trait or disease from their parents, based on whether it is dominant or recessive. 

Autosomal Inheritance

the failure of chromosomes or sister chromatids to separate correctly during cell division, either meiosis or mitosis. This error leads to daughter cells with an incorrect number of chromosomes, resulting in conditions like Down syndrome when an extra chromosome is inherited by a gamete.

Nondisjunction

a specialized cell division process that reduces the chromosome number by half to create four genetically diverse gametes (sperm and egg cells). It's crucial for sexual reproduction because it ensures that when gametes fuse, the resulting offspring has the correct chromosome number and is genetically unique, unlike the identical cells produced through mitosis.

Meiosis

a condition where an organism has three copies of a specific chromosome instead of the usual two

Trisomy

a version of a gene/a gene option

Allele

an allele that shows its effect even if only one copy is present. represented with a capital letter.

Dominant

an allele whose effect is hidden unless two copies are present

Recessive

having two identical alleles for a gene

Homozygous

having two different alleles for a gene

Heterozygous

the genetic label of a trait

Genotype

the physical expression of a trait

Phenotype

when both alleles are fully expressed but separately in the phenotype, sometimes seen as spotting

Codominance

when the phenotype is a blend of two alleles

Incomplete Dominance

the presence of three or more alternative forms of a gene that can occupy the same locus on a chromosome. This happens to a whole population, not just one individual. 

Multiple Alleles

a trait controlled by two or more genes

Polygenic Trait

a diagram used to predict genetic crosses

Punnet Square

a diagram showing how traits are passed through generations

Pedigree Chart

reproduction that requires two parents to create offspring

Sexual reproduction

when DNA overlaps between the homologous chromosomes in meiosis and shares/mixes the DNA

Crossing over

the random segregation of alleles for different genes, resulting in various combinations of traits.

Independent assortment

difference in DNA even between the same organisms

Genetic variation

a cell or organism that contains two complete sets of chromosomes, one inherited from each parent.

Diploid

a cell or organism that has a single set of chromosomes, which is half the number of chromosomes found in diploid cells.

Haploid

inheritance of traits on sex chromosomes

Sex-linked

a genetic cross between two individuals that are each heterozygous for two traits, used to analyze the inheritance patterns of both traits simultaneously

Dihybrid cross

What is a karyotype?

A chart of homologous chromosome pairs

How many homologous pairs do humans have?

23 homologous pairs

What is the purpose of a karyotype?

To pinpoint unusual chromosome numbers in cells

What are homologous chromosome pairs based on?

Length, centromere position, and banding patterns

What are autosomes in humans?

Chromosomes not involved in sex determination

Which homologous pairs are autosomes in humans?


Pairs 1-22

What do sex chromosomes control in humans?


Inheritance of sex characteristics

What is the composition of the 23rd homologous pair in humans?


XY or XX

What does XX represent in terms of sex chromosomes?


Female (♀)

What does XY represent in terms of sex chromosomes?

Male (♂)

What typically causes abnormal karyotypes?


Errors during meiosis

What is nondisjunction?

failure of homologous pairs to separate

What can result from nondisjunction?

Extra or missing chromosomes in gametes

What is trisomy?

having an extra chromosome

What is down syndrome classified as?

Trisomy 21

What causes down syndrome?

Nondisjunction during anaphase 1

What is the incidence of Down Syndrome?

1 in 700 live births

What is unique about Down Syndrome among autosomal trisomies?

Affected individuals can live to see adulthood

What is Turner Syndrome characterized by?


Missing sex chromosomes

What is a consequence of Turner Syndrome

Sterile females

What is a consequence of Turner Syndrome

XXY sex chromosomes

What are the characteristics of Klinefelter Syndrome?

Sterile males with female characteristics

What is Jacobs Syndrome characterized by?

XYY sex chromosomes

What is a common trait of individuals with Jacobs Syndrome?

Typically taller males

How is the XXX condition distinguished?

By karyotype analysis

What is Cru de chat Syndrome caused by?

Deletion of the short arm of chromosome 5

What are the symptoms of Cru de Chat Syndrome?

Severe mental retardation and a small head

What is a characteristic sound of babies with Cru de Chat Syndrome?

Cries that sound like a distressed cat

What are the causes of abnormal karyotypes?

• Mistakes during meiosis

• Nondisjunction

• Translocations

What are the results of nondisjunction?

• Extra chromosomes in some gametes

• Too few chromosomes in others

• Large-scale genetic changes

What are chromosomal mutations?

• Mutations at the chromosome level

• Occur when parts of chromosomes break off

• Rejoin incorrectly during crossing over

What are the four types of chromosomal mutations?

1. Deletions

2. Insertions

3. Inversions

4. Translocations

What are the consequences of chromosomal mutations?


• Abnormal gene distribution to gametes

• Extra copies of genes

• Lack of essential genes

What is a karyotype?

Photomicrograph of a dividing cells chromosomes

What is a karyotype used for?

Used to identify chromosomal abnormalities

How many chromosomes are in humans?

44 autosomes and 2 sex chromosomes

When was the correct number of human chromosomes established?

Not until 1956 due to the clumping of chromosomes in dead tissue

How are chromosomes paired and grouped?

Paired with homologue and grouped according to length and location of centromere

Preparation of a karyotype part 1

Acquire white blood cells

Preparation of a karyotype part 2

Add chemical (colchicine) to break down spindle fibers which effectively halts all cells at metaphase and kills them

Preparation of karyotype part 3

Add distilled water and centrifuge to separate white blood crlls

Preparation of karyotype part 4

Fix with alcohol and stain

Preparation of karyotype part 5

Photograph burst cells using microscope (ones in metaphase) and then enlarge

Preparation of karyotype part 6

Cut out chromosomes and match pairs

Preparation of karyotype part 7

Sort by decreasing soze

Who is this tool (the karyotype) used by?

Genetic counsellors and cytogeneticists

Where is the karyotype used extensively?

In the HGS (Human Genome Study)

What type of chromosomal abnormalities can be detected by using a karyotype?

Sex cells with abnormal number of chromosomes due to nondisjunction (usually causes embryos that are not viable)

Down Syndrome

Usually an extra chromosome in chromosome pair 21. Sometimes can be caused by translocation of chromosome 21-14

Symptoms of Down Syndrome

Mild to severe retardation, flat face, thick tongue & neck, abnormalities of organs, generally early death.

Klinefelter's Syndrome

Extra X Chromosome

Symptoms of Klinefelter's Syndrome

Small breasts, reduced fertility, and long limbs, male

Examples of chromosomal abnormalities

Can also have XXXY and XXXXY

XXX and X (Turner's Syndrome)

Generally normal females with reduced fertility

Deletion

Small pieces of chromosome get deleted during Interphase. Usually lethal.

Duplication

Pieces of chromosome that got deleted are incorporated into its homologue, essentially duplicating that potion of the chromosome

Translocation

Small pieces of chromosome are deleted form one chromosome &I moved to another non-homologus chromosome

Inversion

Small piece of chromosome "flips" in place

What is Meiosis?

It is the process of reduction division in which the number of chromosomes per cell is cut in half throughout the separation of homologous chromosomes in a diploid cell.

What are the two different Meiotic divisions called?

Meoisis 1 and Meiosis 2.

How many daughter cells are produced that are haploid?

Four

What is a diploid number and what is a haploid number?

A diploid number is a term used to a cell that contains both sets of homologous chromosomes. A haploid cell that contains only a single set of chromosomes.

What is the production of egg cells called?

Oogenesis.

What is the production of sperm cells called?

Spermatogenesis.

Gametes

Gametes are egg and sperm.

Tetrads

Tetrads are the two homologous chromosomes that are attached together. Also called synapsis.

Name the different phases of Meiosis 1 in order.

Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, and Interkinesis.

Name the different phases of Meiosis 2 in order.

Prophase 2, Metaphase 2, Anaphase 2, Telophase 2, and Cytokinesis.

Prophase 1

During Prophase 1, each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Metaphase 1

During Metaphase 1, spindle fibers attach to the chromosomes. The homologous chromosomes align at the equator of the cell.

Anaphase 1

During Anaphase 1, the fibers pull the homologus chromosomes toward opposite ends of the cell.

Telophase 1

During Telophase 1, nuclear membranes form, and the the cell separates into two genetically different cells.

Prophase 2

Nuclear envelope fragments, and the spindle forms.

Metaphase 2

Chromosomes align along the equator of the cell.