Notes
Mendel's rules
- Genes come in versions called alleles. Each organism has two alleles per gene.
- Dominant alleles hide recessive ones. Law of segregation: each gamete gets one allele. Law of independent assortment: different genes sort separately.
Vocabulary and Punnett squares
- Genotype = alleles; phenotype = observable trait. Homozygous (TT, tt) vs heterozygous (Tt).
- $\text{Tt} \times \text{Tt}$: $3:1$ phenotypes, $1:2:1$ genotypes. $\text{Tt} \times \text{tt}$: $1:1$. Dihybrid $\text{AaBb} \times \text{AaBb}$: $9:3:3:1$.
Beyond simple dominance
| Pattern | What the heterozygote looks like | Example |
|---|---|---|
| Incomplete dominance | A blend | Pink snapdragons |
| Codominance | Both traits show | Roan cattle, AB blood |
| Multiple alleles | More than 2 alleles in population | ABO blood type |
| Polygenic | Range of phenotypes | Height, skin color |
Sex-linked traits and pedigrees
- Females XX, males XY. X-linked recessive traits (color blindness, hemophilia) are more common in males.
- Pedigrees: squares are males, circles are females, shaded means affected. Recessive traits can skip generations.
- Nondisjunction causes extra or missing chromosomes (Down syndrome = trisomy 21), seen on a karyotype.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Pp x pp gives 1/2 Pp (purple) and 1/2 pp (white).
The $9:3:3:1$ ratio has 1 out of 16 showing both recessive phenotypes.
Daughters get $X^b$ from dad and $X^B$ or $X^b$ from mom, so half are $X^bX^b$.
An AB father can only pass $I^A$ or $I^B$, never $i$, so the child could not be type O.
The heterozygote is a blend of the two parent phenotypes.
Affected parents with a recessive trait (aa) could only have affected children. So the parents must be Aa for a dominant trait, and the child is aa.
Free response
Write your answer first, then open the scoring guide.
Free-response question
In Labrador-like dogs, a gene for coat length has two alleles: short hair (H) is dominant to long hair (h). A breeder has a short-haired male of unknown genotype and wants to know if it carries the long-hair allele.
(a) Describe a test cross the breeder could do.
Show the answer and scoring guide
Cross the short-haired male with a long-haired female (hh), which is homozygous recessive. Then look at the coat lengths of the puppies.
Check off the points you earned:
(b) Use Punnett squares to predict the puppies if the male is HH and if he is Hh.
Show the answer and scoring guide
If HH x hh: all puppies are Hh, so 100% short-haired. If Hh x hh: half Hh (short) and half hh (long), a $1:1$ ratio.
Check off the points you earned:
(c) The litter has 6 puppies, and 2 have long hair. What is the male's genotype? Explain.
Show the answer and scoring guide
The male is Hh. Long-haired puppies are hh, so each must have received an h from the father. An HH father could not pass on h.
Check off the points you earned: