Mendeland Heredity Mendel and Heredity

I. The Origin of Genetics A.Mendel’s Studies of Traits 1. Gregor Johann Mendel-( ) Austrian Augustinian priest and scientist often called the “father of modern genetics” for his study of the inheritance of traits in pea plants. 2. He bred varieties of the garden pea; Pisum sativum in an attempt to understand heredity. 3. Useful Features in Peas – The garden pea is a good subject for studying heredity because: a. it has several traits (7) that exist in two clearly different forms. b. self-fertilization and cross-fertilization are possible. c. the garden pea is easy to cultivate and produces many offspring.

B. Traits Expressed as Simple Ratios 1.Monohybrid Crosses – crosses that involve one pair of contrasting traits ( Hh x Hh) 2. Mendel’s Results a. Mendel observed that contrasting traits appear in offspring according to simple ratios. b. In Mendel’s experiments, only one of the two contrasting forms of a trait was expressed in the F 1 generation. (purebred x purebred) c. The other form reappeared in the F 2 generation in a 3:1 ratio.

Purebred smooth Purebred wrinkled

II. Mendel’s Theory A.A Theory of Heredity 1. Mendel’s Hypothesis a. Different versions of a gene are called alleles. b. An individual usually has 2 alleles for a gene, each inherited from a different parent 2. Mendel’s Findings in Modern Times a. Individuals with the same two alleles for a gene are homozygous b. those with two different alleles for a gene are heterozygous

3. Genotype and Phenotype a. The set of alleles that an individual has is called its genotype. b. The physical appearance of a trait or outward expression of the genotype is called the phenotype. B. The Laws of Heredity 1. The Law of Segregation - the two alleles for a trait separate when gametes are formed (meiosis) 2. The Law of Independent Assortment – two or more pairs of alleles separate independently of one another during gamete formation. (meiosis)

Each allele (S or s, Y or y) can be combined for 4 possibilities. (SY, Sy, sY, sy) when combined with another dihybrid, there are 16 possibilities

III. Studying Heredity A.Punnett Squares 1. Function of Punnett Squares – The results of genetic crosses can be predicted with the use of Punnett squares. 2. One pair of Contrasting Traits – Punnett squares can be used to predict the outcome of a monohybrid cross with one pair of contrasting traits. 3. Determining Unknown Genotypes – A test cross can be used to determine whether an individual expressing a dominant trait is heterozygous or homozygous.

Punnett Square

B. Outcomes of Crosses 1. Probability of a Specific Allele in a Gamete a. The probability of a specific allele in a gamete can be predicted with the use of probabilities. b. For a gene with two alleles, the chance of contributing one allele or the other to the gamete is ½. 2. Probability of the Outcome of a Cross a. The results of a genetic cross can be predicted with the use of probabilities. b. To find the probability that a combination of two independent events will occur, multiply the separate probabilities of the two events.

Probability of a Coin Toss Probability of heads 1 of 2 or ½ Probability of tails 1 of 2 or ½ Probability of heads twice in a row ½ x ½ = ¼ Probability of heads four times in a row ½ x ½ x ½ x ½ = 1/16

C. Inheritance of Traits 1.Pedigrees – A trait’s pattern of inheritance within a family can be determined by analyzing a pedigree. 2. Patterns of Inheritance – Scientists use pedigrees to determine whether a trait is autosomal or sex- linked, dominant or recessive, and heterozygous or homozygous.

The End