A Summary of Mendel's Principles Genes are passed from parents to their offspring. If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another.

Beyond Mendel Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Beyond Mendel Incomplete Dominance – When one allele is not completely dominant over another it is called incomplete dominance. – In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.

Beyond Mendel Codominance – both alleles contribute to the phenotype. – In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. – Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.

Beyond Mendel Multiple Alleles – Genes that are controlled by more than two alleles – An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. – A rabbit's coat color is determined by a single gene that has at least four different alleles.

Beyond Mendel Polygenic Traits – Traits controlled by two or more genes are said to be polygenic traits. – Skin color in humans is a polygenic trait controlled by more than four different genes.

Human Pedigree

A pedigree chart can help trace the genotypes and phenotypes in a family. A pedigree can be used to determine whether people carry recessive alleles.

Pedigree Symbols = Male = Affected Male = Male Carrier = Deceased Male = Marriage = Female = Affected Female = Female Carrier = Deceased Female = Offspring

Using Phenotypes to Determine Genotypes If about the same number of males and females have the gene then the gene is on an autosome. If more males than females have the phenotype, then the gene is on the X-chromosome

Tracing Autosomal Genes A person who does show the trait must be homozygous recessive People without the trait are either homozygous dominant or heterozygous

Tracing Autosomal Genes Two parents that are recessive can only have children that do have the trait Two parents that do not have the trait can have children that have the trait or children that do not have the trait (Homozygous or heterozygous)

Tracing Sex-Linked Genes A male with a sex-linked trait will pass on the allele to all of his daughters and none of his sons If a female has the trait, she must be homozygous recessive

Tracing Sex-Linked Genes Males that have the trait have one recessive allele and one Y chromosome Females who are carriers will not show the trait

Practice Sam and Sean recently got married. They have two children (a boy and a girl). Sean is colorblind. Sean and Sam’s son is colorblind, but their daughter is not colorblind. Sean has two brothers, and neither of them are colorblind. Sam has one sister and one brother. Her brother is colorblind, but her sister is not.

SeanSam 4. What is the genotype of the son? ____________ XcYXcY 1. What is Sean’s genotype? ________________ 2. What is Sam’s genotype? _____________ 3. What is the genotype of the daughter? ____________________ XCXcXCXc XCXcXCXc XcYXcY