chapter 6: Mendel & heredity

Mendel’s Breeding Experiments The garden pea is a good subject for studying heredity for several reasons: Several traits of the garden pea exist in two clearly different forms. The male and female reproductive parts of garden peas are enclosed within the same flower. This allows you to easily control mating. The garden pea is small, grows easily, matures quickly, and produces many offspring. Mendel carried out his experiments in three steps:

Modern terms for Mendel's findings Dominant alleles are indicated by writing the first letter of the trait as a capital letter. Recessive alleles are also indicated by writing the first letter of the dominant trait, but the letter is lowercase. If the two alleles of a particular gene present in an individual are the same, the individual is said to be homozygous. If the alleles of a particular gene present in an individual are different, the individual is heterozygous. In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed. The set of alleles that an individual has is called its genotype. The physical appearance of a trait is called a phenotype.

Black colored fur in mice is a dominant trait over white fur Black colored fur in mice is a dominant trait over white fur. If a pure black mouse mated with a pure white mouse, what would be the outcome?

Punnett Squares A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross. The possible gametes that one parent can produce are written along the top of the square. The possible gametes that the other parent can produce are written along the left side of the square. Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. Punnett squares can be used to predict the outcome of a monohybrid cross (a cross that considers one pair of contrasting traits between two individuals). Punnett squares allow direct and simple predictions to be made about the outcomes of genetic crosses.

Monohybrid Cross: Homozygous Plants

Monohybrid Cross: Heterozygous Plants

Red colored flowers of a certain plant are dominant over yellow colored flowers. The diagram below shows the possible outcomes of the offspring of two hybrid flowers.

More examples 1. The allele for dimples (D) is dominant to the allele for no dimples (d). A man heterozygous for dimples marries a woman who is also heterozygous for dimples. What is the man’s genotype and the woman’s genotype? What is the man’s phenotype and the woman’s phenotype? Do a cross to determine all potential dimple genotypes and phenotypes for the offspring of this man and woman.

2. The allele for hitchhiker’s thumb (h) is recessive to straight thumb (H). If a man and his wife are both homozygous recessive, will any of their offspring potentially have hitchhikers thumb? What is the man’s genotype and the woman’s genotype? What is the man’s phenotype and the woman’s phenotype? What genotype(s) must the offspring have in order to have the phenotypic trait of hitchhiker’s thumb? Do a cross to determine all potential hitchhiker’s thumb genotypes and phenotypes for the offspring of this man and woman. Is it possible for any offspring of the F1 generation to have hitchhiker’s thumb?

3. In humans, free earlobes are dominant over attached earlobes 3. In humans, free earlobes are dominant over attached earlobes. If one parent is homozygous for free earlobes, while the other has attached earlobes can they produce any children with attached earlobes? 4. In humans widow’s peak is dominant over straight hairline. A heterozygous man for this trait marries a woman who is also heterozygous. a) List possible genotypes of their offspring. b) List the phenotypic ratio for their children.

5. In a certain breed of dogs, a gene (L) codes for hair length 5. In a certain breed of dogs, a gene (L) codes for hair length. The dominant trait is short hair and the recessive is long hair. Suppose a heterozygous female dog and a homozygous recessive male dog mate. What is the male dog’s genotype and the female dog’s genotype? What is the male dog’s phenotype and the female dog’s phenotype? What will be the genotypic ratio of the F1 generation? What will be the phenotypic ration of the F1 generation?

6. In fruit flies, the allele for normal wings (V) is dominant to the allele for short wings (v). Suppose two fruit flies heterozygous for the trait are mated. What is the male fruit fly’s genotype and the female fruit fly’s genotype? What is the male fruit fly’s phenotype and the female fruit fly’s phenotype? What will be the genotypic ratio of the F1 generation? What will be the phenotypic ration of the F1 generation?

Quick check! The diagram on the right shows the expected results of a cross between two pea plants. T and t represent the alleles for the tall and dwarf traits, respectively. Use the figure below to answer questions 1–3. 1. What are the genotypes of the plants that were crossed? A. tt on the top; tt along the side B. Tt on the top; tt along the side C. Tt on the top; Tt along the side D. TT on the top; TT along the side 2. What genotypic ratio is expected in the offspring of this cross? F. 1 Tt : 1 tt G. 3 Tt : 1 tt H. 1 Tt : 3 tt J. 1 TT : 1 tt 3. If this cross produced 240 offspring, how many of the offspring would be expected to have the dwarf trait? A. 0 B. 60 C. 120 D. 180

Inheritance of Traits Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder.

Determining Unknown Genotypes Animal breeders often need to know whether an organism with a dominant phenotype is heterozygous or homozygous for a trait. In a test cross, an individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual Like Punnett squares, probability calculations can be used to predict the results of genetic crosses. Probability is the likelihood that a specific event will occur.

Autosomal or Sex-Linked? Scientists can determine several pieces of genetic information from a pedigree: Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Examples: Red-green colorblindness, Male Pattern Baldness, Hemophilia Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait.

Variation Variation within a species is the way that two animals of the same species display different characteristics and/or behavior. There are two types of variation; genetic and environmental.

Some genetic disorders