Honors Biology Unit 10 Review: Heredity and Mendelian Genetics Principles

Mendel bred pea plants in the 1850s and 1860s and found that traits pass down as discrete units, which we now call genes. For most autosomal genes in a diploid organism, an individual carries two alleles, one from each parent. The law of segregation says those alleles separate during meiosis, so each gamete gets one. The law of independent assortment says alleles for different genes sort into gametes independently, as long as the genes are on different chromosomes or far apart on the same one.

A dominant allele (written with a capital letter, like $P$) shows up in the phenotype with just one copy. A recessive allele ($p$) shows up only in a homozygous recessive individual ($pp$). Because $PP$ and $Pp$ plants look identical, you can't read genotype from phenotype. A test cross with a $pp$ plant can reveal the genotype. Any white offspring means the purple parent was $Pp$, but purple offspring alone do not prove it was $PP$.

Punnett squares are probability tools. For independent events (this AND that), multiply the probabilities. For outcomes that can't happen together (this OR that), add them.

Worked example. Two $RrYy$ pea plants are crossed, where $R$ is round and $Y$ is yellow. What fraction of the offspring are wrinkled and yellow? Treat it as two separate monohybrid crosses. From $Rr \times Rr$, $P(rr) = \frac{1}{4}$. From $Yy \times Yy$, $P(Y\_) = \frac{3}{4}$. Multiply them to get $\frac{1}{4} \times \frac{3}{4} = \frac{3}{16}$. That is the 3 in the 9:3:3:1 ratio.

In incomplete dominance, the heterozygote has an in-between phenotype. Red ($RR$) crossed with white ($R'R'$) snapdragons gives pink ($RR'$) offspring. Cross two pinks and you get 1 red : 2 pink : 1 white, so the alleles never actually blended. In codominance, the heterozygote shows both phenotypes fully. Roan cattle have red hairs and white hairs side by side, not pink hairs.

Some genes have multiple alleles in a population, though each person still carries only two. ABO blood type has three alleles. $I^A$ and $I^B$ are codominant, so $I^AI^B$ is type AB. Both are dominant over $i$, so $ii$ is type O.

Genes also interact. In pleiotropy, one gene affects many traits. The sickle cell allele changes hemoglobin. People with two copies can develop sickle cell disease, which can cause anemia, pain and organ damage. In epistasis, one gene controls whether another is expressed. In Labradors, $ee$ dogs are yellow no matter what the $B$ gene says, which changes 9:3:3:1 into 9 black : 3 chocolate : 4 yellow. Polygenic traits like height and skin color are influenced by many genes and the environment. They often show a continuous range of variation, though the distribution is not always bell-shaped.

Sex-linked genes sit on the sex chromosomes, mostly the X. Males (XY) have one X, so a single recessive allele there affects them. That's why hemophilia and red-green color blindness are much more common in males.

Linked genes sit close together on the same chromosome, so they tend to be inherited as a unit. Thomas Hunt Morgan found this in fruit flies when some trait combinations showed up far more often than independent assortment predicts.

Linked genes can still be separated by crossing over. During prophase I of meiosis, homologous chromosomes pair up in synapsis. Non-sister chromatids then swap matching segments at points called chiasmata. Crossing over can produce recombinant chromosomes with allele combinations different from those on either original homolog. The farther apart two genes are, the more likely a crossover lands between them.

Recombination frequency is the percentage of recombinant gametes. In a cross where recombinant offspring can be identified, their percentage estimates that frequency. One percent recombination corresponds to about 1 map unit, or centimorgan, for genes close together. Multiple crossovers can make the observed frequency underestimate distance. The expected maximum is 50%, which looks like independent assortment.

Worked example. An informative fruit fly test cross produces 1,000 offspring, and 170 of them are recombinant. The recombination frequency is $\frac{170}{1000} \times 100 = 17\%$, so the genes are approximately 17 map units apart. To find gene order, compare distances. Say A to B is 8 map units, B to C is 12, and A to C is 20. Since $8 + 12 = 20$, B must sit between A and C, giving the order A, B, C.

Gene maps built this way helped researchers locate disease genes, such as the Huntington's gene on chromosome 4.

A pedigree is a family tree for one trait. Squares are males, circles are females, shaded symbols are affected, and half-shaded symbols are known carriers. Horizontal lines join parents, and vertical lines lead to their children.

Autosomal traits are on chromosomes 1 through 22, so they affect both sexes about equally. An autosomal dominant trait like Huntington's disease often appears in every generation, and affected people usually have an affected parent. New variants and incomplete penetrance can create exceptions. An autosomal recessive trait like cystic fibrosis often skips generations, because two unaffected carriers can have an affected child.

X-linked recessive traits like hemophilia mostly affect males, who usually inherit the allele from a carrier mother. An affected father can't pass an X-linked trait to his sons, since he gives them his Y. So if a pedigree shows father-to-son transmission, X-linkage is ruled out.

To read a pedigree, check whether the trait skips generations, compare how many males and females are affected, look for father-to-son transmission, and then assign genotypes and calculate probabilities.

Worked example. A carrier mother ($X^HX^h$) and an unaffected father ($X^HY$) have a son. What is the chance he has hemophilia? A son always gets the father's Y, so his X comes from his mother. Half of her eggs carry $X^h$, which makes his chance $\frac{1}{2}$. Before you know the sex, the chance that any child is an affected son is $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.