Mendelian Genetics

BSC 2010L Mendelian Genetics

Human Chromosomes • Humans have 46 chromosomes, 23 homologous pairs • Remember that the # of chromosomes differs for different species • When looking at each pair of homologous chromosomes: • 1 came from mom, one came from dad • Each contains the same genes, however an individual can have 2 alternate forms of that gene (alleles)

Alleles • Alleles can be dominant or recessive • Let’s look at eye color • Brown eyes are dominant – B • Blue eyes are recessive – b • Homozygous – having the same alleles • BB or bb • Heterozygous – having different alleles • Bb • Genotype vs Phenotype • Genotype – what alleles does that person have for that gene? • BB or bb or Bb • Phenotype – refers to an individual’s appearance, what is their eye color? • BB or Bb – Brown eyes • bb – blue eyes

Law of Segregation • Each organism contains 2 alleles for each trait, and the alleles segregate during the formation of gametes. Each gamete then contains only 1 allele for each trait. When fertilization occurs, the new organism has 2 alleles for each trait, one from each parent • This is why it is important that 1 of each homologous pair of chromosomes ends up in the gamete at the end of meiosis

Monohybrid cross • Eye color • Bb x Bb (what is the eye color of both these parents?) • Gametes of both these parents: • Looking at the Punnett Square below, these parents have a 75% chance of having a brown eyed child and 25% chance of having a blue eyed child • Genotypic Ratio – 1: 2: 1 (BB:Bb:bb) • Phenotypic Ratio – 3:1 (brown:blue)

Dihybrid Cross • Let’s look at corn • P = purple kernel • p= yellow kernel • S = smooth • s = wrinkled kernel • Think of each kernel as being an individual offspring

Dihybrid Cross • PpSs x PpSs • What are the gametes of these 2 individuals? Remember, one of each letter MUST be in each gamete • Now let’s do a Punnett Square

Dihybrid Cross • Resulting possible offspring: • 9 – purple, smooth • 3 – purple, wrinkled • 3 – yellow, smooth • 1 – yellow wrinkled • = 16 possibilities (count # of boxes in cross) • Dihybrid cross between 2 heterozygous individuals results in a 9:3:3:1 phenotypic ratio

Chi-Square Analysis • When doing these Punnett Squares, will data obtained from sample always follow the calculated ratios? • NO! These are predictions. However, looking at a larger population, one can test and see if deviations from the expected values are just due to chance • We do that by using a statistical tool such as the Chi-Square

Before we go on: • Chi-Square versus Punnett Square • Don’t get these confused!!!! • Punnett squares are actual squares that we use to help us match up chromosomes that are in gametes • Chi-Square • Statistical test – the number is referred to as “x2=“

Chi-Square Analysis • Let’s look at the dihybrid cross we did with the peas a couple of slides back • We expect a 9:3:3:1 ratio • We take an ear of corn and count: • 201 purple, smooth • 70 purple, wrinkled • 89 yellow, smooth • 22 yellow, wrinkled

Chi-Square Analysis • X2 = 5.289 • C-1=3 (# of phenotypes-1, 4-1=3) • Scan across row 3 • p value for x2 value of 5.289 falls between 0.20 and 0.10 • Hypothesis is supported • Our sample fit 9:3:3:1 ratio, differences are due to just random chance

X-linked crosses • With the sex chromosomes, some alleles only occur on the X chromosome • Therefore, males with a recessive gene on their X chromosome will express that trait • Let’s take a look at hemophilia: • XhX x XY

X-linked Crosses • This couple has the following chances with their offspring: • 50% chance girl, 50% boy • 75 % chance of having child with no hemophilia • 25% chance of having boy with hemophilia • 25% chance of having girl who is carrier of hemophilia

Today’s Lab • Continue looking at slides of mitosis and meiosis under microscope from last week • Continue looking at models of mitosis and meiosis • Practice Punnett squares • Observe corn examples • Monohybrid cross • Dihybrid cross • Perform Chi-Square analysis using corn examples

Mendelian Genetics