Genetics and Inheritance

Patterns of Inheritance

Early Theories of Genetics

  • Inherited traits:

    • Can disappear in one generation and reappear in another.

    • Segregate (separate) among offspring.

    • May appear more likely than alternatives (e.g., blond parents having blond children).

Gregor Mendel

  • Austrian monk, born in 1822, known as the father of genetics.

  • Studied heredity in peas because:

    1. Easy to work with, short generation times.

    2. Varieties of true-breeding lines exist (offspring traits resemble parents).

    3. Sexual organs are within the flower:

      • Male and female organs in flower: self-fertilization.

      • Remove male organs, fertilize with pollen from other plant: cross-fertilization.

Mendel’s Experiments

  1. Allowed plants to self-fertilize to ensure progeny had same traits as parents (true-breeders).

  2. Crossed different varieties with different traits (e.g., white flower x purple flower).

  3. Allowed offspring to self-pollinate, allowing traits to segregate; counted offspring exhibiting each trait.

Mendel’s Results

  • P = parental generation, F = filial generation (cross between offspring).

  • True breeders of different types, when crossed, produced offspring of one variety of true breeders (e.g., purple flowers crossed with white flowers produced purple-flowered offspring).

  • When the F1 generation was crossed (purple-flowered x purple-flowered), some purple and some white flowers appeared in a 3:1 ratio (refer to Fig 13.19).

  • Traits: dominant ('takes over') and recessive ('hidden').

  • Self-pollinating F1 generation to get F2 generation:

    1. 141\over4 were always true breeders for the recessive trait.

    2. 141\over4 were always true breeders for the dominant trait.

    3. 121\over2 exhibited the dominant trait but were not true breeders.

    4. Found a 1:2:1 ratio for the F1 x F1 cross.

    • Dominant allele: capital letter (e.g., P for purple flowers).

    • Recessive allele: lower case of dominant allele (e.g., p for white flowers).

    • Each letter: one copy of the gene for each cell.

      • Heterozygous: different, e.g., Pp (one dominant, one recessive).

      • Homozygous: same, e.g., PP (two dominant) or pp (two recessive).

      • Phenotype: what the trait looks like (e.g., purple flower, white flower).

      • Genotype: genetic information in the cell, e.g., PP (homozygous dominant), pp (homozygous recessive), Pp (heterozygous).

Punnett Square

  • Learn how to make a Punnett square.

Mendel’s 1st Law of Heredity: Law of Segregation

  • Alternate traits (genes) separate.

  • Heterozygous individuals can give rise to individuals with recessive or dominant traits.

Test Cross

  • Used to figure out the genotype of a plant.

    1. Test plant of unknown genotype (e.g., purple flower, can be PP or Pp) by crossing it with one with a recessive genotype (e.g., white flower, pp).

    2. Results:

      • If test plant is PP, offspring will be Pp and all purple.

      • If test plant is Pp, offspring will have a 50% chance of being Pp or pp, and half will be white.

Mendel’s 2nd Law of Heredity: Independent Assortment

  • Traits (genes) segregate independently from each other.

  • To test, he crossed plants, observed two traits (genes), and counted the results (Fig 13.16). (Perform the Punnett square on your own).

  • Phenotype ratio: 9:3:3:1 (dominant for both: dominant for one, recessive for the other: recessive for one, dominant for the other: recessive for both).

Genetics: Not Always Clear Cut

  1. Epistasis: Some phenotypes are the expression of more than one gene (both genes must be dominant to express the dominant phenotype). Ex. in corn (fig 13.17)
    E1E2E1 E2
    Starting molecule (colorless) Intermediate (colorless) Anthocyanin (purple pigment)

  2. Continuous Variation: Many traits are the result of polygenes (many genes), resulting in a range of phenotypes (e.g., height in humans).

  3. Pleiotropic Effects: A gene may have multiple effects. Ex., a gene that results in yellow coat color in mice is dominant, but when recessive it causes death to the mouse.

  4. Lack of Complete Dominance: Some alleles are not totally dominant or recessive (e.g., red flower x white flower = pink flower).

  5. Environmental Effects: Some genes are dependent on environmental cues (e.g., artic fox: white coat in winter, brown coat in summer; Himalayan rabbits and Siamese cats: heat-sensitive tyrosinase active in the cold, hence black feet, ears, and snouts).

Chromosomes

  • DNA molecules carrying genetic information about heritable traits.

  • Sexual reproduction: males and females differentiated by a pair of chromosomes:

    1. In humans: 22 pairs are autosomal, 1 pair is the sex chromosome.

    2. Sex chromosomes: X and Y, where Y codes for 'maleness.' Y chromosome is 'missing' some genes present in the X chromosome.

Sex Linkage

  • Some genes, other than sex-genes, are located in the sex chromosomes (e.g., eye color in Drosophila melanogaster - fruit fly).

    • Dominant trait: R = red eye, r = white eye.

    • Female: XX, so XRXR or XRXr (red eyes), XrXr (white eyes), < chances of expressing white (25:75), only females can be carriers.

    • Male: XY, so XRY (red eyes), XrY (white eyes), no heterozygote, > chances of expressing the white-eye phenotype (50:50), see fig 13.12.

Genetic Recombination

  • Genes on the same chromosome can combine with its homologous chromosome, resulting in gene recombination (Fig 13.22).

    1. Chromosome 1: A = red flower, B = tall.

    2. Chromosome 2: a = white, b = short.

    3. No crossing over:

      • Gametes: AB and ab.

      • Crossed with gamete 'ab': offspring will be AaBb (red and tall) or aabb (white and short).

    4. Crossing over takes place:

      • Gametes: Ab and aB.

      • Crossed with gamete 'ab': offspring will be Aabb (red and short) or aaBb (white and tall).

  • Recombination results in offspring being different from parents.

  • Genetic maps of chromosomes: recombination takes place more frequently between genes that are close together, therefore frequency of recombination can be used to make genetic maps. Human genome has been completely sequenced, but the mapping of many traits is still unknown.

Multiple Alleles

  • ABO group: I represents an allele that is codominant (both expressions of the gene are dominant). The genes codes for specialized sugars on the surface of red blood cells, IA = galactosamine and IB = galactose (both can be expressed at the same time)

  • Some genes have more than two possible alleles, such as the ABO group

    1. People can have gametes w/ the following possibilities IA, IB, i (none)

    2. Therefore possible blood types can be IAIA, IAi (A), IB IB, IBi (B), IAIB (AB), or ii (O)

    3. Paternity issues may be resolved by testing blood types, Ex. a child that has A blood and a mother of A blood type (IAIA, IAi) cannot be the son/daughter of a father with B (IB IB, IBi). Why?

Human Abnormalities

  1. Result of non-disjunction (of homologous pairs)

    • one gamete ends up with no copies of the chromosome the other one ends up with two copies.

    • When the one w/ two copies gets fertilized, it ends up with 3 copies resulting in a ‘trisomy’

      • In autosomal chromosomes trisomy 21 results in Down syndrome

      • In sex chromosomes trisomy results in:

        1. XXX: sterile female

        2. XO: Turner syndrome, sterile female, decreased mental abilities

        3. XXY: Klinefelter syndrome, sterile male, decreased mental abilities

        4. OY: Nonviable

  2. See table 13.3 pg. 266, for other human disorders, they may be used in examples for Punnett squares in quiz/test.