Patterns of Inheritance Gregor Mendel Austrian monk In 1860, developed laws of heredity Performed crosses between garden pea plants He concluded that plants transmitted distinct factors (genes) to offspring Genes are found on chromosomes

Chromosomes come in pairs called homologous chromosomes Same length Same centromere location Same gene location (locus) May have different forms of the gene (allele)

Letters on homologous chromosomes stand for alleles Ex. H = widow’s peak h = straight hairline Capital letters represent dominant traits Lowercase letters represent recessive traits Based on his studies, Mendel came up with the law of segregation: Each individual has two factors (genes) for each trait The factors segregate (separate) during the formation of the gametes. Each gamete contains only 1 factor (gene) from each pair of factors (genes). Fertilization results in each new individual having 2 factors (genes) for each trait.

Phenotype: describes what the person actually looks like Ex. Right handed or left handed Purple petals or white petals Genotype: refers to the alleles the chromosomes carry that are responsible for that trait Ex. H = right hand, h = left hand Dominant alleles will mask the expression of the recessive allele when they are together.

Homozygous: two alleles in the pair are the same Ex. AA = homozygous dominant Ex. aa = homozygous recessive Heterozygous: two alleles in the pair are different Ex. Aa The gametes only have one allele for each trait.

One-trait Crosses To see the probability of outcomes of crossing two parents, a Punnett square is used. Monohybrid crosses: crossing of parent genotypes for 1 trait Make a key to indicate parent genotypes Combine all possible gametes using a Punnett square Determine genotypes & phenotypes of all offspring Write results as a ratio, fraction, or percent

The One-Trait Testcross Testcross: the cross of a known homozygous recessive with an unknown genotype to see if the offspring have any dominant traits Example: Both you and your brother are left-handed, but your parents are both right-handed. Right- handedness (H) is dominant over left-handedness (h). What are the genotypes of your parents? Example: My husband and my two kids are left-handed, but I am right-handed. What is my genotype?

The Inheritance of Two Traits Mendel’s law of independent assortment: Each pair of factors (genes) separates independently (without regard to how the others separate). All possible combinations of factors can occur in the gametes. Dihybrid crosses: two different traits crossed together A 16 square Punnett square is used Example: Cross a father who is heterozygous for dimpled cheeks and a cleft chin with a mother who is the same. D= dimples d= no dimples R= cleft chin r= no cleft chin

Pedigrees A pedigree is a chart of a family’s history with regard to a particular genetic trait. Males = squares Females = circles A line between the two represents mating A vertical lines branching off represents offspring Affected individuals are shaded Carriers are half shaded

Incomplete Dominance Incomplete dominance: heterozygote is a mixture of the two alleles because neither allele dominates. Example: Red four o’clock plants mixed with white four o’clock plants produce pink offspring. Cross a red four o’clock plant with a white four o’clock plant. R1R1= red R2R2= white R1R2= pink

Codominance Both alleles show up in the heterozygote Roan cattle are brown and white spotted cows The allele for brown coat color is equally dominant to white coat color Example: R1R1= brown coat R2R2= white coat R1R2= brown & white spots (roan) AB blood type

Multiple Alleles There are several alleles for the same gene that control the inheritance of the trait. A person can only have two of the possible alleles for the trait ABO blood types are an example IA= A antigen on red blood cells IB= B antigen on red blood cells i= neither A or B antigen on red blood cells What are the possible genotypes for a person with A type, B type, AB type, and O type blood? Is it possible for a man who is type A blood and a woman who is type B blood to have a child who is type O blood?

Sex-Linked Inheritance Traits controlled by genes on the sex chromosomes Male sex chromosomes= XY Female sex chromosomes= XX X-linked genes are alleles found on the X chromosome Most sex-linked genes are on X chromosome The male Y chromosome is mostly blank for these diseases Examples of sex-linked diseases: colorblindness, hemophilia, Duchenne’s Muscular Dystrophy

Polygenic Inheritance Two or more sets of alleles determine the trait Each dominant allele codes for a product The more dominant alleles the “stronger” the phenotype Results in a variety of phenotypes Examples: skin color, height, weight, eye color Aa and Bb alleles coding for skin color AABB= very dark skin AaBb= medium color skin aabb= very light skin Environmental factors, such as temperature and nutrition, can affect phenotypes. Examples: height and nutrition, Himalayan rabbits & temperature

Inheritance of Linked Genes All the alleles on one chromosome form a linkage group because they tend to be inherited together. Example: red hair and freckles, blond hair and blue eyes Genes that are farther apart on the chromosome are less likely to be linked because they separate more often in crossing over.