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
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