Mendelian Genetics

How Genetics Began

• A monk named Gregor Mendel first studied how certain traits could be passed on by studying his pea plants.

Heredity (or inheritance): the passing of traits to the next generation

What Mendel did in his experiments:

• He found two plants that were “pure-breds” or true-breeding of certain traits.

• Those two plants were called the parent or “P” generation

• Once he pollinated those two plants, he waited until seeds were produced, planted them and then observed what characteristics they had

• The seeds from the parent plants make up the “F1” generation

Traits in pea plants that Mendel studied

• Allele: two forms of one trait (example #1 : seed color could be yellow or green example #2: plants could be tall or short)

• Dominant traits- the trait that shows through in the organism

• Recessive Trait- the trait that is “masked” (doesn’t show)

Dominance

• Homozygous- an organism with two of the same alleles for a particular trait

• Heterozygous- an organism with two different alleles for a particular trait

*The outward appearance of an organism does not always indicate which pair of alleles is present

Genotype: the organism’s allele pairsPhenotype: the observable characterisitics

(what the organism actually looks like)

Mendel’s Genetic Laws

1. Law of segregation: the 2 alleles for each trait separate during meiosis

2. Law of Independent Assortment: a random distribution of alleles occurs during gamete formation.

Practice: Monohybrid Crosses

• Tongue rolling is a dominant trait. Suppose both parents can roll their tongues and are heterozygous (Tt) for the trait. What possible phenotypes and genotypes could their children have?

Practice: Monohybrid Crosses• A person with a widow's peak (Pp) is married to a

person with a straight hairline (pp), what percentage of their children will have a straight hairline?

Practice: Monohybrid Crosses

Two people who are both heterozygous for the widow's peak trait are married. What percentage of their children will have a straight hairline?

Review: Monohybrid Crosses

• Gregor Mendel is out working in his pea garden one day and decides to cross a plant that is homozygous for round seeds (RR). He crosses this plant with one that is heterozygous for round seeds (Rr).

• Create a Punnett Square that shows the possible genotypes that could be seen in the F1 generation.

What if you wanted to look at TWO traits at one time???

You do a dihybrid cross:

• Example: Mendel crossed two pea plants, both were heterozygous for yellow color and round seeds (YyRr) and wanted to see the chances that the offspring would have green, wrinkled seeds?

Complex Patterns of Inheritance

• Incomplete Dominance• Codominance• Mulitple Alleles• Epistasis• Sex Determination

Incomplete Dominance• Heterozygous phenotypes is an intermediate phenotype

between two homozygous phenotypes

• Example: A red-flowered plant (RR) is crossed with a white-flowered plant (rr) produces pink flowered plants that are heterozygous (Rr).

• How do you know for sure you’re seeing Incomplete Dominance? Look for a “new color phenotype” that’s in the middle of the parents.

Codominance

• Both alleles are expressed in the heterozygous condition

• Example: A white cow mates with a red bull. The offspring has a mixture of red and white fur

• How do you know? Both alleles are expressed

Codomanice: Roan coloration

Multiple Alleles

• More than two alleles can influence the trait.

• Example: Human blood groups (three forms of blood alleles: IA IB i). A, B, AB, or O

Sex Determination

• There are two types of sex chromosomes: X and Y

• Individuals with two X chromosomes are female

• Individuals with an X and a Y chromosome are male

Karyotype: Chromosomes are arranged by size and structure

• Pairs 1-22 are called autosomes • The 23rd pair are called sex chromosomes: They control

what sex the organism will be

• What is the sex of the karyotype on this slide?