variations in heredity ppt

Geneticists can use the testcross to

determine unknown genotypes

• _{A testcross is the mating between an individual of unknown genotype}

and a homozygous recessive individual.

• _{A testcross can show whether the unknown genotype includes a}

recessive allele.

• _{Mendel used testcrosses to verify that he had true-breeding varieties}

of plants.

• _{The following figure demonstrates how a testcross can be performed}

to determine the genotype of a Lab with normal eyes.

Genes on the same chromosome

tend to be inherited together

• _{Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in}

the F₂ generation. What they found was an example of linked genes, which

• _{are located close together on the same chromosome and} • _{tend to be inherited together.}

Figure 9.17-2

The Explanation: Linked Genes Parental diploid cell PpLl Most gametes Most offspring Meiosis Fertilization Sperm

3 purple long : 1 red round

Not accounted for: purple round and red long

Epistasis

A gene at one locus affects the expression of a

gene at another locus

• Example of polygenic inheritance.

• _{Labrador gene for coat color BB for Black and bb for chocolate brown.}

A single gene may affect many

phenotypic characters

• _{Pleiotropy occurs when one gene influences multiple characters.} • _{Sickle-cell disease is a human example of pleiotropy.}

• _{This disease affects the type of hemoglobin produced and the shape of red}

blood cells and causes anemia and organ damage.

• Sickle-cell and nonsickle alleles are codominant.

• _{Carriers of sickle-cell disease have increased resistance to malaria.}

A single character may be

influenced by many genes

• _{Many characters result from polygenic inheritance, in which a single}

phenotypic character results from the additive effects of two or more genes on a single phenotypic character.

• _{Human skin color is an example of polygenic inheritance.}

Multifactoral Traits

Genes may predispose us to certain

conditions, but environmental factors

also play a critical role.

There is a genetic component to many diseases:

• _Alcoholism

• _{Heart Disease}

• _Diabetes

The environment affects many

characters

• _{Many characters result from a combination of heredity and the}

environment. For example,

• _{skin color is affected by exposure to sunlight and}

• _{heart disease and cancer are influenced by genes and the environment.}

• _{Identical twins show that a person’s traits are the results of}

• _{genetics and}

• _{the environment.}