Notes
Meiosis
Meiosis makes haploid gametes ($n$) from a diploid cell ($2n$) in two divisions.
- Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.
- Genetic variation comes from crossing over (prophase I), independent assortment of homologous pairs (metaphase I), and random fertilization.
- Nondisjunction (chromosomes failing to separate) gives gametes with an extra or missing chromosome, causing conditions like trisomy 21.
Mendelian genetics
- Law of segregation: the two alleles for a gene separate into different gametes.
- Law of independent assortment: genes on different chromosomes assort independently.
- Monohybrid cross $Aa \times Aa$: 3:1 phenotype ratio. Dihybrid cross $AaBb \times AaBb$: 9:3:3:1.
- Probability rules: multiply for "and" (independent events); add for "or" (mutually exclusive outcomes).
Beyond simple dominance
- Incomplete dominance: blended heterozygote (red × white gives pink).
- Codominance: both alleles show (AB blood type).
- Sex-linked traits on the X chromosome show up more often in males ($X^aY$), who have only one X.
- Linked genes are close on the same chromosome and are inherited together unless crossing over separates them. Recombination frequency (%) ≈ map distance in map units.
- Polygenic traits (height, skin color) and environmental effects (hydrangea color depends on soil pH) also shape phenotypes.
- Mitochondrial DNA is inherited from the mother.
Chi-square in genetics
Compare observed and expected offspring: $\displaystyle \chi^2 = \sum \frac{(o - e)^2}{e}$, $df = $ number of phenotype classes $- 1$. If $\chi^2$ is greater than the critical value at $p = 0.05$, reject the null hypothesis that the data fit the expected ratio.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Meiosis I separates homologs; meiosis II separates sister chromatids.
$\displaystyle \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.
$\displaystyle \frac{3}{4} \times \frac{3}{4} = \frac{9}{16}$.
The heterozygote has an intermediate phenotype.
Sons get their X from the mother, who passes $X^h$ half the time.
Independent genes give 50% recombinants; 8% means they're close together.
Crossing over swaps segments between homologous chromosomes.
Mitochondria come from the egg.
Free response
Write your answer first, then open the scoring guide.
Free-response question
In a cross expected to give a 3:1 ratio, 160 offspring were 104 purple and 56 white.
(a) Calculate the expected numbers of each phenotype.
Show the answer and scoring guide
Purple: $\displaystyle \frac{3}{4}(160) = 120$. White: $\displaystyle \frac{1}{4}(160) = 40$.
Check off the points you earned:
(b) Calculate $\chi^2$.
Show the answer and scoring guide
$\displaystyle \frac{(104 - 120)^2}{120} + \frac{(56 - 40)^2}{40} = 2.13 + 6.4 = 8.53$.
Check off the points you earned:
(c) The critical value for $df = 1$ at $p = 0.05$ is 3.84. State a conclusion.
Show the answer and scoring guide
$8.53 > 3.84$, so reject the null hypothesis. The difference from a 3:1 ratio is not due to chance alone, so the trait may not follow simple dominance (or the genes may be linked or selection may be acting).
Check off the points you earned: