Introduction to Genetics Chapter 11

What is genetics? The scientific study of heredity

Gregor Mendel Born in 1822 in Czechoslovakia. Became a monk at a monastery in 1843. Taught biology and had interests in statistics. Also studied at the University of Vienna

Mendel continued After returning to the monastery he continued to teach and worked in the garden. Between 1856 and 1863 he grew and tested over 28,000 pea plants

Mendel’s Peas Easy to grow. Easily identifiable traits Can work with large numbers of samples

Mendel’s experiments The first thing Mendel did was create a “pure” generation or true-breeding generation. He made sure that certain pea plants were only able to self pollinate, eliminating unwanted traits. He did this by cutting away the stamen, or male part of each flower

Genes and dominance Trait : a characteristic Mendel studied seven of these traits After Mendel ensured that his true-breeding generation was pure, he then crossed plants showing contrasting traits. He called the offspring the F1 generation or first filial.

What will happen when pure yellow peas are crossed with pure green peas? All of the offspring were yellow. Hybrids = the offspring of crosses between parents with contrasting traits

What did Mendel conclude? Inheritance is determined by factors passed on from one generation to another. Mendel knew nothing about chromosomes, genes, or DNA. Why? These terms hadn’t yet been defined.

What were Mendel’s “factors” The ‘factors” that Mendel mentioned were the genes. Each gene has different forms called alleles Mendel’s second principle stated that some alleles are dominant and some are recessive.

Mendel’s second cross He allowed the F1 generation to self-pollinate thus producing the F2 generation. Did the recessive allele completely disappear? What happened when he crossed two yellow pea hybrid (F1) plants?

Results: ¾ of the peas were yellow, ¼ of the peas were green. During the formation of the sex cells or gametes, the alleles separated or segregated to different gametes. (pollen and egg)

Probability The likelihood of a particular event occurring. Chance Can be expressed as a fraction or a percent. Example: coin flip.

Punnett Square Developed by Reginald Punnett. A diagram used to show the probability or chances of a certain trait being passed from one generation to another.

Reading Punnett squares Gametes are placed above and to the left of the square Offspring are placed in the square. Capital letters (Y) represent dominant alleles. Lower case letters (y) represent recessive alleles.

Punnett square example

genotypes Homozygous = when an organism possesses two identical alleles. ex. YY or yy Heterozygous = when an organism possesses different alleles. ex. Yy

Phenotype vs genotype Genotype The genetic makeup Symbolized with letters Tt or TT Phenotype Physical appearance of the organism Expression of the trait Short, tall, yellow, smooth, etc.

Probability and statistics No one event has a greater chance of occurring than another. You cannot predict the precise outcome of an individual event. The more trials performed, the closer the actual results to the expected outcomes.

Punnett square review:

Independent Assortment The two factor cross. Example: color and shape of peas. F1 cross to produce the F2 generation Ex RRYY x rryy Round yellow mated with wrinkled green Offspring would all be hybrid for both traits (RrYy)

What is independent assortment? Alleles separate independently during the formation of gametes.

The dihybrid cross Punnett square on board:

Mendel’s death Mendel published his paper on heredity in 1866. The scientific community saw little if any importance in his work. Mendel died in 1884 with no recognition for his contributions to genetics.

Some exceptions to Mendel’s principles: Some alleles are neither dominant nor recessive. Many traits are controlled by more than one gene (polygenic traits)

Incomplete dominance A situation in which neither allele is dominant. When both alleles are present a “new” phenotype appears that is a blend of each allele. Alleles will be represented by capital letters only.

Japanese four-o-clock flowers Red flower plant genotype = RR White flower plant genotype = WW Pink flower plant genotype = RW

What happens when a red flower is crossed with a white flower? According to Mendel either some white and some red or all offspring either red or white. All are pink

Codominance When two alleles both appear in the phenotype. Usually signified using superscripts. example: color of hair coat in cattle. crcr = red hairs cwcw = white hairs crcw = roan coat (mixture of both colors)

Roan cattle inheritance

Multiple allele inheritance When two or more alleles contribute to the phenotype. Human blood types: A,B,O and AB A and B are codominant to each other. Both A and B are dominant over O.

Human Blood types: TYPE A Allele = IA Blood cells have small antigens on the surface.

TYPE B Allele = IB Cells coated with type B antigens

TYPE AB genotype = IAIB Blood cells contain both types of antigens Known as universal recipient

TYPE O Allele = i No antigens on the surface of the blood cells Known as universal donor

6 different genotypes IAIA IAIB IBIB IBi IAi i i Type A Type AB Type B Type O

How common are the different blood types? 45 % 40 % 4 % 11 %

Sample Problem: A man with type AB blood marries a woman with type B blood whose father has type O blood. What are the chances that they have a child with type A blood? Type AB?

Polygenic traits Traits controlled by two or more genes. Examples: Human height, eye and skin color

Rediscovery of Mendel’s work Around the turn of the century (early 1900’s) many scientists “rediscovered” Mendel’s work 1908 – Garrod 1902 – Sutton 1910 – Morgan

Thomas Hunt Morgan 1866-1945 Born in Kentucky, professor of Biology at Columbia U. Worked with fruit flies (drosophila) Nobel Prize in Medicine (1933)

Why the Fruit Fly? Can work with large numbers of flies easily Produce many offspring Short reproductive cycle Only four pairs of chromosomes

Meiosis A method of cell division similar to mitosis. 2 main differences: There are two divisions to produce 4 daughter cells The cells produce contain ½ the chromosomes as the original cell

Chromosome number All cells of an organism contain a specific number of chromosomes. Most cells are diploid (2n) meaning they have two copies of each chromosome

Events of meiosis I During prophase I, each chromosome pairs with its homologous chromosome to form a tetrad

Crossing-over Crossing-over: an exchange of genetic material between sister chromatids Results in greater variation

Meiosis II Neither cell replicates its chromosomes. Each cell splits (similar to mitosis) Produces four daughter cells. Animation

Gametogenesis Literally means “creation of gametes” Egg and sperm

2 types: Spermatogeneis & Oogenesis

Net result: Spermatogensis 4 mature sperm Each sperm has exactly half the number of chromosomes as the father. Oogensis 1 mature ova or egg. Each egg has exactly half the number of chromosomes as the mother.

Gene Linkage Are genes “linked” to each other on chromosomes? Morgan found that many genes are linked together. It was determined that chromosomes, not genes, assort independently during meiosis.

Gene Maps First developed by Sturtevant in 1911. The farther apart two genes are, the more likely they will be separated in meiosis.

Assignment: worksheet Pages 283-284 1-10, 13, 14, 17, 18, 24