Intro to Genetics

• Genotype (gene)

  Phenotype (trait)
• Alleles: Different versions of the same gene
• Alleles can be heterozygous or homozygous
• Organisms can have heterozygous alleles for one gene and homozygous alleles for another
• The rules of mathematics and probability determine the chances of particular outcomes (i.e., combinations of genes).
• every time an organism makes haploid gametes (eggs or sperm), only one of the two copies of each gene (either maternal or paternal) winds up in each gamete – it’s a “flip of the coin”.
• Metaphase I : Homologous pairs line up in center of cell, with bivalents oriented randomly with respect to each other.
• For each chromosome, it’s random whether mom’s set of genes or dad’s will get pulled towards one side of the dividing cell vs. the other during Anaphase I
• Two (or more) events are independent if the outcome of one doesn’t affect the outcome of the others.
• In meiosis I, the direction in which the long dark blue chromosome homolog moves is independent of which direction the short dark blue chromosome moves.
• Inheritance

  Each individual outcome (which chromosome ends up in each daughter cell) is unique and based on random distribution during metaphase I and II.
• The basis of inheritance lies in two key principles

  1. The segregation of alleles of a single gene
  2. The independent assortment of different genes on different chromosomes
• The segregation of alleles of a singlegene

  • Individuals inherit two copies of each gene, one from the mother and one from the father, and when individuals form gametes, the two copies separate equally in each gamete
• The independent assortment of different genes on different chromosomes

  • The two copies of each gene segregate into gametes independently of the two copies of another gene
• Segregation reflects the separation of homologous chromosomes during Anaphase I of Meiosis
• Independent assortment of genes on different chromosomes reflects the fact that nonhomologous chromosomes can orient in either of 2 ways that are equally likely
• These principles were discovered in 1800’s by crossing true breeding pea plants strains
• True breeding: the appearance of the offspring is identical to the parents.

  • Pea plants have 7 easily observed traits (phenotypes).
• Mendel concluded:

  • There must be different alleles (e.g.yellow, green) for a given trait (e.g. seed color)
• Mendel concluded:

  • Certain alleles of a trait appear to have dominance over others, as evidenced by the F1 generation (yellow over green)
• Mendel concluded:

  • Yet, the dominated (“recessive”) alleledoesn’t just disappear, as evidenced by itsre-emergence in the next (F2) generation
• F2 generation: always found a 3:1 ratio
• Mendel’s conclusions about the 3:1 ratio in gen F2:

  • F1 must inherit a dominant allele from one parent (in this case, yellow) and a recessive allele (green) from the other (heterozygous)
• Mendel’s conclusions about the 3:1 ratio in gen F2:

  • During their transmission to the F2 generation (meiosis), dominant (yellow) and recessive (green) alleles must separate from each other, such that any given F2 can inherit one or the other allele from each parent (rather than inheriting both from both parents). In other words, the principle of segregation
• Mendel’s conclusions about the 3:1 ratio in gen F2:

  • Explains why green reappears in ~ ¼ ofF2’s: these inherit recessive allele fromboth parents (homozygous recessive)
• Offspring (F1 generation) will always be heterozygous for seed color gene, having inherited an A from one parent and an a from the other
• By the Principle of Segregation, whenheterozygous F1’s form their own gametes, A allele and a allele will get separated, so ½ gametes will contain A and the other ½ a
• In the formation of the F2 generation, the gametes from the F1 parents combine at random.
• Dihybrid crosses can illustrate second principle of inheritance: Independent Assortment