Inheritance Patterns Simple dominance, incomplete dominance, codominance, sex-linkage polygenic inheritance

Applying Mendel’s Laws We can use Mendel’s Law of Segregation to predict the outcome of possible crosses of individuals First, we need some terminology Genotype: the alleles an organism has (e.g. I have the genotype bb because I have two copies of the blue eyes allele) Phenotype: the characteristics an organism displays (e.g. my phenotype is blue eyes)

Terms Homozygous – a genotype that has two identical alleles (bb is a homozygous genotype because both alleles are the same) Heterozygous – a genotype that has two different alleles (Bb is a heterozygous genotype because the alleles are different)

Mendel & Inheritance Patterns Mendel observed only a particular type of inheritance pattern His pea plants had an inheritance pattern of simple dominance.

Mendel & Inheritance Patterns Type of alleles in simple dominance: Dominant allele: always appears in the phenotype, whether it is part of a homozygous or heterozygous genotype e.g. if a person is heterozygous for polydactyly (having more than 5 fingers) – Pp – they will have an extra finger Recessive allele: two copies of the allele must appear in the genotype (it must be homozygous) for the trait to appear in the phenotype tongue rolling – you will only not be able to roll your tongue if you have genotype rr

Simple Dominance: Using the terms We use capital letters for dominant alleles We use lower case letters for recessive alleles The letters for the alleles should be the same (e.g. F for purple flowers allele, f for white flowers allele)

Monohybrid crosses If we are looking at the inheritance of only one trait, we are looking at a monohybrid cross We can predict the outcome of a monohybrid cross when there is simple dominant-recessive inheritance A Punnett grid helps us to do this.

Monohybrid crosses, continued We know from Mendel’s Law of Segregation, that gametes contain only one allele for each trait So, if a purple pea plant is heterozygous for flower color, its gametes will either have a F allele (purple) or a f allele (white) A homozygous white pea plant’s gametes can only have a f allele (white) We can show this in a Punnett grid

Monohybrid crosses, continued When we fill in the grid, the center boxes show us the possible genotypes of the offspring

Monohybrid crosses, continued The genotypic ratio for the outcome of this cross is 1:1, Ff:ff The phenotypic ratio is also 1:1, purple:white

Incomplete Dominance For some genes, neither gene is fully dominant over another. In incomplete dominance, the F1 phenotype will be somewhere in between the two parental phenotypes.

Incomplete Dominance In this type of inheritance pattern, the heterozygote produces less red pigment, so the F1 generation flowers are pink. If the F1 offspring are crossed, what would you predict the outcome would be?

Codominance Codominance is similar to incomplete dominance. In a heterozygous individual, however, each of the alleles will produce their own distinct product. Example: Blood type

Codominance: Blood type Blood types differ due to proteins that are present on the outside of red blood cells. In a person who is a heterozygote for the A and B alleles, both the protein for type A and the protein for type B will be produced.

Codominance: Blood type Predict what the possible outcomes are for a child who has one parent with type A blood and one parent with type AB blood.

Sex-linkage When genes are carried on the sex chromosomes, unique patterns of inheritance are created. Example: Hemophilia is a recessive trait carried on the X chromosome. Predict the outcome of a cross between a normal man and his wife, who is a carrier (heterozygous) for hemophilia

Dihybrid Crosses If we want to predict the outcome of a cross involving two genes, we use a dihybrid cross. Example: round yellow seeds x green wrinkled seeds.

Polygenic Inheritance Traits aren’t always controlled by only one gene. Polygenic Inheritance happens when more than one gene affect a single phenotypic characteristic. Example: Skin color in humans is controlled by at least three different genes (and probably more)

Review: Follow the link from Edmodo to the Mendelian Genetics problem sets from the Biology Project Do the monohybrid crosses, dihybrid crosses, and the first sex-linked inheritance problem set. Use the tutorials to help you understand problems with which you have difficulty.