Genetics

Review Chromosomes are paired (homologous) in a 2N organism. Each chromosome contains many genes Homologous chromosomes carry the same genes. Homologous pair of chromosomes

Review Genes can have different versions of the same trait. These versions are called alleles. Homologous chromosomes separate into different gametes. alleles

Alleles - Dominant and Recessive Each parent will donate one allele. Alleles can be physically apparent (dominant) or physically hidden (recessive). Mendel worked with traits that had only two alleles of each trait, one dominant and one recessive. Alleles Mother Father

Mendel’s Laws The Law of Segregation The Law of Dominance each pair of alleles separates during meiosis (gamete formation) The Law of Dominance When two alleles differ, one can control the trait - Dominant the other can be hidden - Recessive The Law of Independent Assortment Pairs of genes segregate into gametes randomly and independently (due to random separation of homologous pairs.

How Can We Demonstrate these Laws? Mendel recorded phenotypes ( form of the trait that he “saw”) Based on outcomes from matings (called crosses) over several generations, he could determine the genotypes (the actual genetic makeup for that trait) of previous generations Homozygous - both alleles for a trait are the same Heterozygous - alleles are not identical.

Monohybrid Crosses Studying the inheritance of a single trait while ignoring all other traits Start with 2 “pure parents” P Generation GG (only green) gg (only yellow) F1 generation All green heterozygous

Determining Genotype (1 Trait) PARENT green yellow F1 4 All green Cannot see the yellow in this generation Self- fertilized F2 Self fertilization of this generation produces a different pattern for homozygous vs. heterozygous Yellow returns

Using a Punnett Square instead Grid for Organizing Genetic Information Shows Probability that a combination will occur g g Parent 2 Gg G Gg 1) Determine Parent alleles 2) Place alleles on edge of square 3) Combine alleles inside the boxes 4) Determine genotypes and phenotypes Gg Gg Possible offspring genotypes for F1 Parent 1

Heterozygous cross G g G g Phenotypes vs. Genotypes The recessive trait will show ONLY in the absence of the dominant (G) trait. G g GENOTYPE G g GG : Gg or gG : gg 1 : 2 : 1 GG Gg PHENOTYPE Green Yellow 3 : 1 gG gg

Test Crosses Purpose - To distinguish between a homozygous dominant and a heterozygous dominant Cross the unknown with a known homozygous recessive. Create two Punnett squares. Cross a homozygous dominant with a homozygous recessive Cross a heterozygous dominant with a homozygous recessive How are the patterns different?

Heterozygous + Homozygous Recessive Phenotypes vs. Genotypes The recessive trait will show ONLY in the absence of the dominant (G) trait. What will happen? g g GENOTYPE GENOTYPE G g GG :Gg or gG : g : : PHENOTYPE Green Yellow :

Predicting any single outcome Given Mendel’s Laws, the probability of an outcome for any one combination can be estimated. Can Use coins to estimate probability See Exercise on Probability.

Probabilities and Proportions Should approximately match. Are only estimates of what SHOULD happen. Can be easily changed If “randomness” is compromised.

Dihybrid crosses RY RY ry Allow the study of two traits at a time. Make a P cross (RRYY x rryy) Cross F1 plants among themselves (RrYy x RrYy) List all gametes on a 4 x 4 Punnet Square RY RY ry

Intermediate Inheritance Mendel’s study Dominant and recessive Many genes have intermediate inheritance Heterozygous phenotype not like either homozygous phenotype

Types of Intermediate Inheritance Incomplete Dominance Heterozygote phenotype is “inbetween “ the two homozygous phenotypes EXAMPLE: Red + White = Pink Flowers BUT: Gametes will contain either Red or White! No dominant phenotype so only capital letters with superscripts are used. Red = FrFr White = FwFw Pink = FrFw

Types of Intermediate Inheritance Codominance Both alleles express themselves fully EXAMPLE: Blood types A, B, AB

Polygenic Traits Trait is controlled by two or more gene pairs Eye color, skin, hair Tone amount, and distribution of pigments are all factors. Each is controlled by a different gene pair, for each attribute (color, skin, hair)

Multiple alleles Three or more alleles in a trait Blood type IA IB and i Both IA and IB are dominant over I IA and IA are codominant. NOT THE SAME as polygenic A single pair of genes with > 2 alleles.

Environmental Stress on Genetics Seasonal Changes( external changes) Turning on “beard growth (internal changes) Genes can be turned on and off.