BSC 2010L

Human Chromosomes Humans have 46 chromosomes, 23

homologous pairsRemember that the # of chromosomes

differs for different speciesWhen 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)

Alleleso Alleles can be dominant or recessive

o Let’s look at eye coloro Brown eyes are dominant – Bo Blue eyes are recessive – b

o Homozygous – having the same alleleso BB or bb

o Heterozygous – having different alleleso Bb

o Genotype vs Phenotypeo Genotype – what alleles does that person have for that gene?

o BB or bb or Bbo Phenotype – refers to an individual’s appearance, what is their eye

color?o BB or Bb – Brown eyeso 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 parentThis 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 colorBb 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 cornP = purple kernelp= yellow kernelS = smooths = 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

o Resulting possible offspring:o 9 – purple, smootho 3 – purple, wrinkledo 3 – yellow, smootho 1 – yellow wrinkledo = 16 possibilities (count # of boxes in cross)

o 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 SquareDon’t get these confused!!!!

○ Punnett squares are actual squares that we use to help us match up chromosomes that are in gametes

○ Chi-SquareStatistical 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 AnalysisX2 = 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 chromosomeTherefore, males with a recessive gene on

their X chromosome will express that traitLet’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% boy75 % chance of having child with no

hemophilia25% chance of having boy with hemophilia25% 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 crossDihybrid cross

Perform Chi-Square analysis using corn examples