Introduction to Genetics
Every living thing – plant or animal, microbe or human being – has a set of characteristics inherited from its parents. Since the beginning of recorded history, people have wanted to understand how that inheritance is passed from generation to generation. The delivery of characteristics from parent to offspring is called heredity. Genetics is the scientific study of heredity.
Austrian Monk. Born 1822 in Czech Republic. Worked at monastery and taught high school. Tended the monastery garden. Grew peas and became interested in the traits that were expressed in different generations of peas.
Mendel cross pollinated peas and observed the traits of the offspring. A trait is a specific characteristic of an individual.
P1 Generation- the first parental generation in a genetic cross. F1 Generation- the first generation of offspring in a genetic cross. F2 Generation- the second generation of offspring in a genetic cross.
The offspring of crosses between parents with different traits are called Hybrids. When Mendel crossed plants with different traits he expected them to blend, but that’s not what happened at all. All of the offspring had the character of only one of the parents.
Mendel drew two conclusions which formed the basis of our current understanding of inheritance. First: An individual’s characteristics are determined by factors that are passed from one parental generation to the next. Genes are the factors that are passed from parent to offspring. Alleles are the different forms of a gene.
Genes occur in pairs. Alleles are the different forms of genes that have contrasting effects on a trait. You inherit 2 alleles for each characteristic – one from each parent.
Second: The principle of dominance – some alleles are dominant and others are recessive. Dominant alleles cover up the recessive gene. It is the allele expressed. It is represented with a capital letter. Ex. T = tall Recessive alleles get covered up in the presence of a dominant allele. It is the allele hidden. It is represented with a lower case letter. Ex. t = short
Mendel wanted to answer another question. Q: Had the recessive alleles disappeared? Or where they still present in the F 1 plants? To answer this he allowed the F 1 plants to produce an F 2 generation by self pollination
P 1 Parental TallShort All Tall F1F1 F2F2 3 tall : 1 short 75% tall 25% short
The recessive traits reappeared! Roughly 1/4 of the F 2 plants showed a recessive trait
The reappearance indicated that at some point the allele for shortness had been separated from the allele for tallness. Mendel suggested that the alleles for tallness and shortness in the F1 plants were segregated from each other during the formation of sex cells or gametes. When each F1 plant flowers, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes – those with the allele for tallness and those with the allele for shortness
Mendel kept obtaining similar results, he soon realized that the principals of probability could be used to explain the results of genetic crosses. Probability is the likelihood that a particular event will occur. The way in which alleles segregate is random like a coin flip.
Diagram used to determine genetic crosses. A way to visualize the results of a genetic cross is by using a Punnet Square, a chart that shows all possible genotypes of a cross.
If you do not know the following vocabulary words you will fail miserably
Organisms that have 2 identical alleles for a trait. Homo means same. Ex.) TT, tt
Have two different alleles for a trait. Hetero means different. Ex.) Tt
Physical characteristics – (words) Ex.) tall
Genetic make-up - (letters) Ex.) Tt, TT, tt
Mendel wondered if alleles segregate during the formation of gametes independently. Does the segregation of one pair of alleles affect the segregation of another pair of alleles? For example, does the gene that determines whether round or wrinkled in shape have anything to do with the gene for color? Must a round seed also be yellow?
All heterozygous9:3:3:1 Ratio
Genes that segregate independently do not influence each others inheritance.
The inheritance of biological characteristics is determined by individual units known as Genes. In organisms that reproduce sexually, genes are passed from parents to offspring. In cases in which 2 or more forms of a gene are present, some forms of the gene may be dominant or recessive. In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another.
When one allele is not dominant over another. Ex. Four o’clock flowers. The heterozygous phenotype is somewhat in-between the two homozygous phenotypes. Example: homozygous red flowered snapdragon crossed w/ a homozygous white flowered snapdragon; all F 1 offspring will be pink.
When both alleles contribute to the phenotype of an organism Ex.) Speckled Chickens
When more than two possible alleles exist in a population Ex.) blood type IAIA IBIB ii Dominant Recessive
Human Blood Types PhenotypeGenotype A B AB O I A I A or I A i I B I B or I B i I A I B ii
Traits controlled by two or more genes Ex.) eye color, skin color
The characteristics of any organism, is not only determined by the genes it inherits. Characteristics are determined by interactions between genes and the environment. Ex.) genes may affect a plants height but the same characteristic is influenced by climate, soil conditions and availability of water.
Human hair is inherited by incomplete dominance. Human hair may be curly (CC) or straight (cc). The heterozygous genotype (Cc) produces wavy hair. Show a cross between two parents with wavy hair.
A man is suing his wife on grounds of infidelity. The man claims that the child is blood type O and therefore must be fathered by someone else. Can he use this evidence in court if he and his wife both have heterozygous B genotypes? Show the cross of the two parents.
1. Each organism must inherit… a single copy of every gene from each of its parents. 2. When an organism produces its own gametes… these two sets of genes must be separated from each other so that each gamete contains just one set of genes.
Every species has a certain number of chromosomes in each cell.
Ex.) fruit fly 8 chromosomes 4 from mom, 4 from dad Ex.) Humans 46 chromosomes 23 from mom, 23 from dad
Chromosomes that each have a corresponding chromosome from the opposite sex parent. Chromosomes occur in pairs. These pairs are called homologous chromosomes. The pair has the same size, shape, and similar traits. (You inherit one from each parent)
23 pairs = 46 chromosomes
Human Chromosomes
A cell that contains both sets of homologous chromosomes (2N). ◦ Body cells
A cell that contains only a single set of chromosomes (1N) ◦ Sex cells (gametes)
A haploid sperm and a haploid egg join to form a diploid zygote.
A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. ◦ Makes sex cells.
Meiosis I Meiosis II
1 st Division: the homologous chromosomes are separated into 2 cells. 2 nd Division: the chromatids of each chromosome are separated into separate cells.
In Meiosis, two nuclear divisions take place instead of one.
prior to meiosis I, each chromosome is replicated. The cells then begin to divide similar to mitosis.
Each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad - has 4 chromatids.
When chromosomes exchange portions of their chromatids and results in the exchange of alleles.
Leads to new combinations of alleles The homologous chromosomes separate, and 2 new cells are formed Although each cell now has 4 chromatids something is different. Because each pair of homologous chromosomes was separated, neither of the daughter cells has two complete sets of chromosomes that it would have in a diploid cell The two sets have been shuffled
The two cells produced by meiosis I now enter a second meiotic division. Unlike the 1st division, no chromosomes are replicated. Each cell’s chromosomes has 2 chromatids.
2 chromosomes line up in the center of each cell.
The paired chromatids separate.
Forms 4 daughter cells each with 2 chromatids These 4 daughter cells are now haploid (N) – just 2 chromosomes each
A total of 4 sex cells are produced, each with half the number of chromosomes.
In male animals, the haploid gametes produced by meiosis are called sperm. In some plants they are called pollen.
In females, generally only one of the cells produced by meiosis is involved in reproduction. This female gamete is called an egg. The other 3 cells that do not receive as much cytoplasm as the egg are called polar bodies.
Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells
46 23 MitosisMeiosis
When genes are located on the same chromosome they are inherited together (Linkage). It’s the chromosomes that assort independently not individual genes.
When genes are formed on the same chromosome, this does not mean that they are linked forever. Crossing over during meiosis sometimes separates genes that had been on the same chromosome onto homologous chromosomes. Cross over events occasionally separate and exchange linked genes and produce new combinations of alleles.
A: Generates genetic diversity
1911 Alfred Sturtevant hypothesized that the further apart genes were, the more likely they were to be separated by a crossover in meiosis. the rate at which linked genes were separated and recombined could then be used to produce a “map” of distances between genes.
Shows the location of each gene.
Genetic variation between parents and offspring gives some. offspring a better chance of surviving in a changing environment. CROSSING OVER during meiosis increases genetic variation.
Fertilization then development
The End