Heredity
Heredity
Basic Concepts
Heredity: The passing of traits from parents to offspring through genetic information.
Involves the transmission of genetic material, including genes, from one generation to the next.
The basic unit of heredity is the gene, which carries the instructions for specific traits.
Key Terminology
CHROMOSOME: A thread of DNA, made up of a string of genes.
GENE: A length of DNA that is the unit of heredity and codes for a specific protein; a gene gets copied and passed on to the next generation.
ALLELE: Any of two or more alternative forms of a gene.
HAPLOID NUCLEUS: A nucleus containing a single set of unpaired chromosomes (e.g., sperm and egg).
DIPLOID NUCLEUS: A nucleus containing two sets of chromosomes (e.g., in body cells).
Understanding Chromosomes and Genes
Genes: Control the characteristics of living organisms.
Location: Genes are carried on the chromosomes.
Chromosome Pairs: Chromosomes are in pairs in diploid organisms, one from each parent.
Gene Pairs: Genes are also in pairs as a result of being located on chromosome pairs.
Genes controlling the same characteristics occupy identical positions on corresponding chromosomes.
Gene Pairs and Trait Determination
Gene pairs control one characteristic, but they do not always control it in the same way.
Example: Coat color in mice, where one gene might produce black fur and its partner might produce brown fur.
Dominance: The gene for black fur is dominant to the gene for brown fur.
Genetic Representation
Genes are represented by letters.
Example: The gene for black fur is given the letter B, and the gene for brown fur is given the letter b.
Dominant genes are represented in capital letters, recessive genes in lowercase.
Possible combinations: BB, bb.
Alleles Explained
Alleles are alternative forms of the same gene.
Example: B and b are alleles of the gene for coat color.
B is the dominant allele.
b is the recessive allele.
Surrogacy and Genetic Relationships
Gestational Surrogacy: A medical treatment where a woman bears a child for another woman. It involved In vitro fertilization (IVF).
In Vitro Fertilization (IVF): A process where the egg of the mother (intended mother) and the sperm of the father are fertilized in a lab before transferring it to the uterus of the surrogate mother.
The egg of the mother and the sperm of the father are fertilized in a lab before transferring it to the uterus of the surrogate mother.
Fertility medications stimulate egg production.
Eggs and sperm are collected, and healthy ones are selected.
The sperm and egg are fertilized and stored in a laboratory dish.
Eggs are monitored to confirm fertilization and cell division.
Embryos are formed and then transferred into the uterus of the surrogate mother.
If the process is successful, implantation occurs.
Genetic Relationship in Surrogacy
Traditional Surrogacy: The surrogate mother is the biological mother of the child. This involves the insemination of the surrogate mother's egg with sperm, either from the intended father or a sperm donor.
Gestational Surrogacy: The surrogate mother does not contribute genetically to the child. An embryo is created using the intended parents' or donors' genetic material through IVF and then transferred to the surrogate mother's uterus.
Examples of Heredity and Inherited Traits:
Eye Color: Determined by genes passed down from parents.
Hair Color: Influenced by different combinations of genes.
Blood Type: Determined by specific genes (ABO system: A, B, AB, and O) inherited from parents.
Inherited Diseases: Genetic disorders passed down through generations, such as cystic fibrosis, sickle cell anemia, hemophilia, and Huntington's disease.
Height: Influenced by a combination of genetic and environmental factors.
Skin Pigmentation: Influenced by genes involved in the production of melanin.
These examples illustrate the significant role of heredity in determining physical and genetic traits.
Genetic Traits and Their Regulation
Genetic traits are transferred from parents to offspring through genes in reproductive cells.
A gene is a segment of DNA found on chromosomes, which contains the code for a protein.
Some genetic traits, such as earlobe attachment, the presence of dimples, and the appearance of eyelid creases, are mostly regulated by a few pairs of genes.
Some traits, such as height, intelligence, weight, and skin color, are influenced by many genes and environmental factors.
Chromosomes and Genetic Information
Chromosomes: Structures found in the nucleus of cells that carry genetic information in the form of genes. They are thread-like structures made up of DNA tightly coiled around proteins called histones.
Humans typically have 46 chromosomes, organized into 23 pairs.
Autosomes: (chromosome pairs 1 to 22) contain genes that determine various traits and characteristics, excluding those related to sex.
Sex Chromosomes: The 23rd pair, which determine an individual's biological sex. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Chromosome Numbers in Different Species
Different species have different numbers of chromosomes in each somatic cell.
Examples:
Dog: 78
Horse: 64
Chimpanzee: 48
Human: 46
Cat: 38
Fly: 12
Mosquito: 6
Potato: 48
Tomato: 24
Rice: 24
Red bean: 22
Corn: 20
Papaya: 18
Onion: 16
Cucumber: 14
A genome is the complete set of chromosomes in each cell of a multicellular organism.
The number of chromosomes in an organism is determined by its genetic makeup and evolutionary history.
Humans and other living organisms have different numbers of chromosomes due to various factors, including evolutionary divergence, genetic mutations, and genetic rearrangements over time.
Evolutionary Divergence and Genetic Mutations
Divergent Evolution: Common ancestor adapts different traits.
Example 1: Dog species descend from a wolf.
Example 2: Zebras, donkeys, and horses are related.
Mutation: Any change in the nucleotide sequence as a result of a failure of the system to revert the change.
For each chromosome pair, one copy comes from each parent.
Rearrangement, deletion, insertion, and translocation.
Chromosomes vary in number and shape among living organisms.
Human Chromosomes and Sex Determination
In each human cell, there are 46 chromosomes or 23 pairs.
Chromosomes 1-22, the same in both men and women, are called autosomes that control several genetic traits.
Chromosome 23, sex chromosomes, differs between females and males.
Females have two X chromosomes (XX), while males have one X and one Y chromosomes (XY chromosomes).
In sex chromosomes of men, the X chromosome is the same as those of females but the Y chromosome is smaller.
Therefore, a person's sex is determined by sex chromosomes.
Karyotyping is the process by which photos of chromosomes under a microscope are paired and ordered to examine the number and size of chromosomes.
Examining Chromosomes through Karyotyping
Karyotyping is a laboratory procedure that allows a physician to examine a patient's set of chromosomes.
"Karyotype" also refers to the actual collection of chromosomes being examined.
Examining these chromosomes through karyotyping allows your physician to determine whether there are any abnormalities or structural problems.
Chromosome Structure
A chromosome is a long, threadlike structure composed of chromatin, a complex of DNA and proteins.
It is found in the nucleus of eukaryotic cells and plays an important role in carrying and transferring genetic information.
Before cell division, chromosomes replicate their DNA to make identical copies.
As a cell prepares to divide, it makes a copy of each of its chromosomes.
The two copies of a chromosome are called sister chromatids, joined at the centromere.
Chromatin packs the DNA into a compact unit that fits within the nucleus.
Under the microscope in its extended form, chromatin resembles a string of beads.
The beads are called nucleosomes.
Each nucleosome is composed of DNA wrapping around proteins called histones.
DNA or deoxyribonucleic acid is a molecule that carries genetic information organized into units called genes.
Structures Carrying Genetic Information
Chromosome: Made up of long strands of tightly coiled (twisted) DNA wrapped around proteins.
Chromatin: The 'string-like' material that makes up chromosomes, made up of DNA and proteins.
Histones: Proteins that act as a spool around which chromatin wraps, packing large amounts of chromatin into the nucleus.
Nucleosomes: The chromatin and protein unit, like "beads on a string."
DNA: A large molecule that contains the instructions for making proteins in an organism, shaped like a twisted ladder (double helix).
The length of DNA from one cell is meters.
The length across a cell's nucleus which holds your DNA is cm.
Genes: Small pieces of DNA; individual genes carry instructions to make specific proteins.
There are approximately 20,000 genes in the human genome.
Base Pairs: Two chemical bases joined together. DNA has a total of four chemical bases. The base pairs form a "step of the DNA ladder."
The four nucleotide bases found in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).
Adenine pairs with thymine, and cytosine pairs with guanine. These pairs are held together by hydrogen bonds.
There are 3.2 billion base pairs in the human genome.
Gregor Johann Mendel and the Laws of Inheritance
Gregor Johann Mendel, an Austrian monk and botanist, discovered the fundamental laws of inheritance, which is the primary foundation of genetics.
He performed experiments on planting garden peas (Pisum Sativum L.) to investigate patterns of inheritance.
Garden peas are appropriate characteristics that suit the experiments as follows:
They have distinct in genetic traits.
They are complete flowers.
It is easy to control their reproduction because their flowers can self-pollinate.
They grow fast and yield large amounts of offspring.
They have a short lifespan.
A complete or perfect flower is a plant that has completely of four parts: sepals, petals, stamens, and pistils.
Mendel's Experiments
Mendel studied the inheritance of seven different features in garden peas by crossing them.
Features included:
Seed shape (round, wrinkled)
Seed color (yellow, green)
Flower color (purple, white)
Pod shape (inflated, constricted)
Pod color (yellow, green)
Flower position (axial, terminal)
Plant height (tall, short)
Mendel first established pea lines with clearly opposite features, such as tall and short height.
He planted and aided in self-pollination for the garden pea plants until all of the offspring were pure-breeding (produce offspring identical to the parents).
Mendel removed immature stamens to prevent self-pollination.
Pollen grains of the white flower of a pea plant were transferred to the stigma of the purple flower of another pea plant.
The pollen grains developed into pods. These seeds were planted into offspring plants.
The first filial generation plants (F₁) self-fertilized and their seeds were planted after that.
705 pea plants with purple flowers and 224 pea plants with white flowers were obtained
The ratio between pea plants with purple flowers and those with white flowers is 3:1.
The result of the experiment was all of the offspring of crosses between purebred pea plants (F₁) looked like only one of the parents (purple flowers).
Mendel called the expressed parental trait the dominant trait.
F₁ generation might have some hidden traits, so Mendel crossed two F₁ pea plants to find the answer. The crossing two plants of the same genotype is the way to look at a single trait, which is today called a monohybrid cross.
The result was F had purple and white flowers. The hidden trait is called the recessive trait.
Expression
Round seed expressed planted and yielded 5,474 plants bearing round seeds, while 1,850 bearing wrinkled seeds.
Yellow seed expressed planted and yielded 6,022 plants bearing yellow seeds, while 2,001 bearing green seeds.
Hybrid plants with purple flowers bore 705 plants with purple flowers and 224 plants with white flowers.
Hybrid plants with inflated pods gave 882 plants with inflated pods and 299 plants with constricted pods.
Hybrid plants with green pods bore 428 plants with green pods and 152 plants with yellow pods.
Hybrid plants with flowers along the stems gave 651 plants with flowers along the stems and 207 plants with flowers on the top the stems.
Long hybrid plants yielded 787 long plants and 277 short plants.
Gene Transmission and Alleles
Based on Mendel's findings, an organism has units that control inherited traits.
Such units are arranged in pairs, and each member of the pair is separately found in reproductive cells.
Genes on chromosomes are genetic units that regulate such inherited traits.
One gene codes for one characteristic of an organism.
Genes come in variant forms. Each of these forms is called an allele.
Alleles Defined
Alleles are the different possibilities for a given trait.
Every trait has at least two alleles (one from the mother and one from the father).
Examples: Eye color - Brown, blue, green, hazel
A = Brown Eyes, a = Blue Eyes
B=Green Eyes, b = Hazel Eyes
Genes
Genes are segments of DNA
Each gene contains information about a certain trait
Genes are transcribed and translated by the cell to make proteins
Proteins create a visible phenotype
Example: One gene might code for eye color. The gene is used by cells to make proteins which create green pigment in our eyes.
Homologous Chromosomes and Allele Expression
Each pair of chromosomes has two identical alleles.
Dominant and recessive traits can be expressed by a pair of homologous chromosomes.
One member of the pair is obtained from one parent and the other from the other parent.
The homologous chromosome pairs are of the same length, centromere position, and pattern.
Most genes have two alleles. Dominant alleles are expressed over recessive alleles.
Recessive alleles are only expressed when no dominant allele is present.
So, the characteristics of an organism are expressed by the dominant alleles.
Length, Centromere Location
replication
Homozygous vs. Heterozygous
Homozygous: A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote.
Heterozygous: A diploid organism is heterozygous at a gene locus when its cells contain two different alleles (one wild-type allele and one mutant allele) of a gene. The cell or organism is called a heterozygote.
Genotype and Phenotype
In most sexually reproducing organisms, each gene of each individual has two alleles.
A pair of alleles is called a genotype, and the expressed characteristic of an organism is called a phenotype.
A pair of alleles can be represented by two letters. Dominant alleles are represented by capital letters and recessive alleles by lowercase letters.
Alleles represented by two uppercase (e.g., TT) or lowercase letters (e.g., tt) exhibit a homozygous genotype, which may be either a dominant or recessive trait.
Two different alleles, represented by one uppercase and one lowercase letter (e.g., Tt), express a heterozygous genotype that expresses a dominant trait over a recessive one.
Predicting Inheritance
The study of genetic inheritance can be used to predict what next generations look like.
Example
If we cross a hybrid tall plant and a purebred short plant, let's see what are the genotypes and phenotypes of their offspring (given that the allele for tall stems is represented by T, the allele for short stems is represented by t, and the dominant allele is completely dominant the recessive allele).
Genotypes of parental plants.
Genotype of F generation
The genotype ratio between Tt and tt is 50:50.
The phenotype ratio between the tall-stemmed plants and short-stemmed plants is 50:50.
Punnett Square
The Punnett Square is another method that can be used to determine the genotype and phenotypes of offspring from genetic crosses to see a single trait.
We can construct a Punnett Square by making a grid or table and putting the genotype of one parent across the top and that of the other parent down the left side.
We then write the row and column-head letters across or down into the empty squares and find a ratio of the possible genotype.
The Punnett Square is named after Reginald C. Punnett who invented this method to determine the probability of the genotype of offspring.
Example
Genotypes of Female Parental cat (BB, Bb, bb) Male Parental cat (b) = B b\
Male
B BB Bb\
b Bb bb\The genotype ratio between BB, Bb, and bb is 1:2:1.
The phenotype ratio between black-haired cats and white-haired cats is 3:1.
Example 2: Cross two individuals- one that is homozygous recessive and the other has the dominant phenotype, but had a mother with the recessive phenotype
Genotypes (Bb,bb) (B,b) Dominant with the mother Phenotype\
BB Bb\
bb bb
Cell Division in Organisms
Our body is always making new cells to grow, reproduce, and replace old cells.
The process by which new cells are made is called cell division.
Karyokinesis (nuclear division) and cytokinesis (cytoplasmic division) are two steps in the cell division, which occur simultaneously
Cell division in eukaryotes can be divided into mitosis and meiosis.
Karyokinesis is the process of nuclear division that occurs during cell division. It involves the separation and distribution of replicated chromosomes into two daughter nuclei. An example of karyokinesis is the mitotic division of somatic cells.
Cytokinesis is the process of cytoplasmic division that follows karyokinesis (nuclear division) during cell division. It involves the separation of the cytoplasmic components and the formation of two distinct daughter cells. An example of cytokinesis is the division of animal cells through a process called cleavage.
Mitosis
Cell Division process by which a cell divides into 2 new cells
Why do cells need to divide?
Living things grow by producing more cells, NOT because each cell increases in size
Repair of damaged tissue
If cell gets too big, it cannot get enough nutrients into the cell and wastes out of the cell
Mitosis process
Mitosis is the cell division that produces new body cells to replace dead or damaged cells so that an organism can grow. The process produces two genetically identical daughter cells. The number of chromosomes remains the same as their parent cell.
Interphase
*Cells prepare for division by synthesizing certain materials and replicating their DNA. Chromosomes make identical copies of themselves. The nuclei of the cells become larger and the nucleoli are clearly seen.
Prophase
*Chromosomes condense and their DNA is tightly coiled up. Replicated chromosomes can be seen as separate units, pairs of chromatids. As long as chromatids keep attached to each other at the centromere, they are known as sister chromatids. The centrosomes in cytoplasm begin to produce spindle fibers.
Metaphase
*Spindle fibers, ropelike structures, attach themselves to the centromere of sister chromatid pairs. The fibers line the pairs up in the center of the cell. This area is known as the metaphase plate.
Anaphase
*Spindle fibers contract and separate sister chromatids from each other. The sister chromatids are pulled to two opposite poles of the cells.
Telophase
*The separate sister chromatids are now known as chromosomes. Their DNA becomes loose and new nuclear membranes and nucleoli are formed. Each nucleus contains four chromosomes identical to the parent cell. The cytoplasm of the cells then divides in two to produce two daughter cells.
Meiosis
Meiosis is the cell division that produces haploid gametes (having a copy of each chromosome) from diploid cells (having two copies of each chromosome). The whole process produces four genetically different daughter cells with a half the number of chromosomes as the parent cell. Meiosis consists of two rounds: meiosis I and meiosis II.
Following is meiosis I
Cells prepare for division by synthesizing some materials and replicating DNA. Chromosomes make identical copies of themselves.
Prophase I
Chromosomes condense. Similar chromosomes pair up and become homologous chromosomes. The sections of DNA cross over (get swapped) to exchange genetic materials.
Metaphase I
Spindle fibers attach themselves to the centromere of homologous chromosomes and line them up at the metaphase plate.
Anaphase I
Spindle fibers contract and separate sister chromatids from each other. The sister chromatids are pulled to two opposite poles of the cells. This results in two groups of chromosomes with two chromatids each.
Telophase I
New nuclear membranes and nucleoli are formed. Cytoplasm divides to produce two cells with the haploid number of chromosomes. Sister chromatids remain together.
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