Mendelian Patterns of Inheritance Chapter 11
Law of segregation Each individual has two factors for each trait. The factors segregate (separate) during the formation of the gametes. Each gamete contains only one factor from each pair of factors. Fertilization gives each new individual two factors for each trait.
One-Trait Inheritance • Alleles—alternative form of gene • Form at the same locus on homologous chromosomes. • Dominant allele—allele that exerts it phenotypic effect in the heterozygote • it masks the expression of the recessive allele. • Recessive allele—allele that exerts its phenotypic effect only in the homozygote • Its expression is masked by a dominant allele.
One-Trait Inheritance Gene locus—alleles occur on a homologous pair of chromosomes at a particular location. Homozygous—an organism has two identical alleles at a gene locus. Heterozygous—an organism has two different alleles at a gene locus.
One-Trait Inheritance • Genotype—genes of an organism for a particular trait or traits. • Usually designated by letters—BB or Aa • Phenotype—visible expression of a genotype. • Brown eyes or attached earlobes.
Punnett square Punnett square—grid used to calculate the expected results of simple genetic crosses.
Laws of Probability Allow us to calculate the probable results of one-trait genetic crosses. The chance of an event that can occur in two or more independent ways is the sum of the individual chances.
Practice Problems 1. In rabbits, if B=dominant black allele and b= recessive white allele, which of these genotypes (Bb, BB, bb) could a white rabbit have?
Practice Problems 2. In humans, freckles is dominant over no freckles. A man with freckles reproduces with a woman without freckles. What chance did each child have for freckles?
Practice Problems 3. In horses, trotter (Tt) is dominant over pacer (tt). A trotter is mated to a pacer, and the offspring is a pacer. Give the genotype of all the horses.
One-Trait Testcross • Testcross—cross between an individual with the dominant phenotype and an individual with a recessive phenotype. • Results in phenotypic ratio indicates whether the dominant phenotype is homozygous or heterozygous.
Practice Problems 1. In horses, B=black coat and b=brown coat. What type of cross should be done to best determine whether a black-coated horse is homozygous dominant or heterozygous?
Law of Independent Assortment Each pair of factors segregates independently of the other pairs. All possible combinations of factors can occur in the gametes.
Practice Problems • For each of the following genotypes, give all possible gametes, noting the proportion of each gamete for the individual. • A. TtGG • B. TtGg • C. TTGg
Practice Problems • For each of the following, state whether a genotype (genetic makeup of an organism) or a type of gamete is represented. • A. Tg • B. WwCC • C. TW
Human Genetic Disorders Autosome—any chromosome other than a sex (X or Y) chromosome. Carrier—heterozygous individual who has no apparent abnormality but can pass on an allele for a recessively inherited genetic disorder.
Patterns of Inheritance Autosomal dominant—an individual with the alleles AA or Aa has the disorder. Autosomal recessive—only individuals with the alleles aa have the disorder. A pedigree chart show the pattern of inheritance for a particular condition.
Patterns of Inheritance • A pedigree chart show the pattern of inheritance for a particular condition. • Square is designated for a male • Circle is designated for a female
Autosomal dominant disorders Affected children will have at least one affected parent. Heterozygotes (Aa) are affected Two affected parents can produce an unaffected child. Two unaffected parents will not have any affected children.
Autosomal Recessive disorders Affected children can have unaffected parents. Heterozygous (Aa) have a normal phenotype. Two affected parents will always have affected children. Affected individuals with homozygous dominant mates will have unaffected children. Close unaffected relatives who reproduce are more likely to have affected children if they have joint affected relatives. Both males and females are affected with equal frequency.
Autosomal Recessive Disorders • Tay-Sachs Disease • Demographics— • Eastern Europeans/ Jewish decent • French Canadians, Cajuns (Louisiana) • Old Order Amish in Pennsylvania • 1 in every 300 people who are not of these backgrounds is a carrier for Tay-Sachs
Autosomal Recessive Disorders • Tay-Sachs Disease • Progressive fatal genetic condition that affects the nerve cells in the brain. • Lack a specific protein called hexosaminidse A. • Causes a fatty substance (GM2 ganglioside) to build up in the brain.
Autosomal Recessive Disorders • Tay-Sachs • Development