Mendel & Genetic Variation Chapter 14

What you need to know! The importance of crossing over, independent assortment, and random fertilization to increasing genetic variability

Mendel Crossed true-breeding (homozygous) parents Observed 3:1 ratios in the F2 generation of monohybrid crosses for several traits Developed first laws of genetics

Mendel – Law of Segregation Hypothesis for 3:1 ratio: 1. Alternative versions of genes account for variations in inherited characters (alleles) 2. For each character, an organism inherits two alleles, one from each parent 3. If the two alleles at a locus differ then the dominant allele determines the organism’s appearance

Mendel – Law of Segregation Hypothesis for 3:1 ratio: 4. Law of segregation: two alleles for a heritable character separate during gamete formation and end up in different gametes  Homologous chromosome separation during meiosis I

Mendel – Law of Independent Assortment Dihybrid crosses revealed a 9:3:3:1 ratio in offspring Law of Independent Assortment: alleles separate independently of other alleles (for different genes)  We now know this is only true for genes on different chromosomes

Probability Mendel’s observation of 3:1 ratios in monohybrid crosses and 9:3:3:1 ratios in dihybrid crosses forever linked genetics and probability Rule of multiplication: the probability of two or more independent events all happening is calculated by multiplying their individual probability Rule of addition: the probability of two or more mutually exclusive events occurring is calculated by adding up their individual probabilities

Genetic Variation Key to the survival of a species Only species that have a varied gene pool are able to adapt through natural selection Sources: 1. Mutations 2. Crossing Over 3. Mendel’s law of segregation: shuffling of maternal and paternal chromosomes during Meiosis 1

Probability – Law of Segregation Example for an organism with 6 chromosomes Generate all 8 variations of gametes for the following individual (use colors) Homologous Pair 1: X X Homologous Pair 2: X X Homologous Pair 3: X X Key: MaternalPaternal

Probability – Law of Segregation X XXXXXXX XXXXXXX XXXXXXX XXXXXX X XXXXXXX XXXXXXX XXXXXXX XXXXXX

Rule of multiplication: a gamete can only have one of two possible chromosomes from a single homologous pair ( ½ ) Rule of multiplication: there where three homologous pairs each of which had a ( ½ ) chance ( ½ x ½ x ½ ) = 1/8  Or ½ n ; where n = # homologous pairs How many combinations are possible when humans make gametes? What is the likelihood of having a sibling with the same chromosome combination in humans?