Biology 1 · Study notes
Mendelian genetics describes how traits pass from parents to offspring through the units of heredity we call genes. Gregor Mendel's experiments with pea plants in the 1800s revealed rules of inheritance that still underpin plant breeding, animal husbandry, and genetic counseling. This topic builds on how chromosomes behave during meiosis and gamete formation, and it sets up the next topic, where genes are examined as molecules of DNA. Here, genes are treated only as units of heredity.
Every trait is controlled by genes, and alternate forms of the same gene are alleles. In Mendel's pea plants, the gene for height has a dominant allele for tall and a recessive allele for dwarf; a dominant allele hides the effect of its recessive partner. An individual with two identical alleles is homozygous, and one with two different alleles is heterozygous. The genotype is the pair of alleles an individual carries, while the phenotype is the trait that actually appears.
A monohybrid cross follows one trait. Crossing two true-breeding parents, the P generation, produces an F1 generation that is all heterozygous and all tall. Crossing two F1 plants produces an F2 generation with a 3:1 phenotypic ratio of tall to dwarf and a 1:2:1 genotypic ratio. This result is the evidence for the law of segregation, which states that the two alleles of a gene separate during gamete formation so that each gamete carries one allele. A test cross crosses an individual with a dominant-looking phenotype to a homozygous recessive individual; if any offspring show the recessive trait, the unknown parent was heterozygous.
A dihybrid cross follows two traits at once, such as seed shape and seed color in peas. Crossing plants that are true-breeding for round yellow seeds with plants true-breeding for wrinkled green seeds gives an F1 generation that is all round and yellow, because round and yellow are dominant. The F2 generation shows a 9:3:3:1 phenotypic ratio: 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. This ratio is the evidence for the law of independent assortment, which states that alleles of different genes sort into gametes independently of one another.
| Feature | Monohybrid cross | Dihybrid cross |
|---|---|---|
| Traits followed | One | Two |
| Classic parents | Tall crossed with dwarf | Round yellow crossed with wrinkled green |
| F1 phenotype | All dominant | All dominant for both traits |
| F2 phenotypic ratio | 3:1 | 9:3:3:1 |
| Law it demonstrates | Segregation | Independent assortment |
1. Write the genotypes of both parents, for example RrYy and RrYy.
2. Determine the gametes each parent can make; each gamete gets one allele of each gene.
3. Make a 4 by 4 Punnett square and fill in all sixteen offspring genotypes.
4. Group the offspring by phenotype and reduce the counts to the 9 to 3 to 3 to 1 ratio.
5. State the phenotypic ratio as your answer.Picture a pocket holding one red coin and one white coin, standing for a parent with two different alleles. To make a gamete, the parent hands out one coin chosen at random, and the other stays in the pocket. A child receives one coin from each parent, so it may end up with two reds, one of each color, or two whites. That random handing out of single copies is the law of segregation, and it is why the classic three-to-one ratio appears when heterozygous parents are crossed.
With complete dominance, the heterozygote looks exactly like the dominant homozygote, which is what Mendel saw in tall versus dwarf peas. With incomplete dominance, the heterozygote shows an intermediate phenotype: crossing red snapdragon flowers with white ones gives pink F1 offspring, and the F2 generation splits 1:2:1, one red, two pink, one white. With codominance, both alleles are fully expressed at the same time, as in roan cattle, which show both red and white hairs, and in people with the AB blood type.
The human ABO blood group is a classic case of multiple alleles, a gene with more than two allele forms. Three alleles exist, IA, IB, and i. IA and IB are codominant with each other, and both are dominant over i. The four phenotypes are A, B, AB, and O, produced by the genotypes IA IA or IA i, IB IB or IB i, IA IB, and ii. Blood type matters in transfusions because the body attacks donated blood of incompatible types.
| Pattern | Appearance of heterozygote | Example |
|---|---|---|
| Complete dominance | Same as dominant homozygote | Tall pea plants |
| Incomplete dominance | Intermediate blend | Pink snapdragons |
| Codominance | Both traits visible at once | AB blood type |
Common Mistake: Assuming a heterozygote always shows one parent's trait. In incomplete dominance the heterozygote is a blend, and in codominance both alleles show at once. Always ask what the cross actually produced rather than assuming full dominance.
Think of red paint and white paint. With complete dominance, one color covers the other completely, so the result is just red. With incomplete dominance, the paints mix into pink, a blend of the two. With codominance, the paints stay separate and you see red stripes and white stripes side by side. The same three rules describe how alleles combine in living things.
Human body cells carry 46 chromosomes: 44 autosomes and two sex chromosomes. Females have two X chromosomes, and males have one X and one Y chromosome. Eggs always carry an X, while sperm carry an X or a Y with equal frequency, so each child has a 50 percent chance of each sex. The roughly equal numbers of males and females at birth are the sex-linkage version of a monohybrid cross.
Sex-linked genes are carried on a sex chromosome, most often the X. Because a male has only one X chromosome, he has no second copy to mask a recessive allele, so X-linked recessive traits appear far more often in males than in females. Classic examples are red-green color blindness and hemophilia. A female needs two copies of the recessive allele to be affected, and with one copy she is a carrier. An affected male passes his X chromosome to all of his daughters, who become carriers, and never to his sons, because sons receive his Y. A carrier mother and a normal father have sons with a 50 percent chance of being affected and daughters with a 50 percent chance of being carriers. X-linked dominant conditions are rarer and appear in every generation, and Y-linked genes pass only from father to son.
Common Mistake: Thinking a son inherits an X-linked trait from his father. Sons get their X chromosome from their mother and their Y from their father, so any X-linked allele in a son came from his mother.
A boy is like a worker with a single copy of the instruction manual. If that one copy has a mistake, there is no backup copy to catch it, so the mistake shows up. A girl has two copies, so one good copy can cover for a damaged one. That is why color blindness and hemophilia show up mostly in boys.
A pedigree is a family tree chart that shows who has a trait across several generations. Squares are males, circles are females, filled symbols are affected individuals, and half-filled symbols are carriers of a recessive condition. A horizontal line connects parents, and vertical lines lead down to their children; generations are numbered with Roman numerals and individuals with Arabic numerals. Doctors use pedigrees to trace inherited diseases and to predict the risk for a future child.
Reading a pedigree is a detective task. If two unaffected parents have an affected child, the trait is recessive, because each parent must be hiding one copy of the allele. If every affected child has an affected parent, the trait is dominant. To tell an autosomal trait from an X-linked recessive one, look for father-to-son transmission: it rules out X linkage, since a father gives his sons his Y, not his X. A strong male bias among affected individuals suggests X-linked recessive inheritance.
1. Label each person by sex and by affected status.
2. Check whether unaffected parents produced an affected child; if so the trait is recessive.
3. Check whether every affected child has an affected parent; if so the trait is dominant.
4. Look for father-to-son transmission; if present the trait is not X-linked.
5. Compare the number of affected males with affected females to choose between autosomal and X-linked.A pedigree is like a family photo album that only shows whether each person has freckles. You can look back through grandparents, parents, and children to see how freckles skip a person or reappear. If two parents without freckles have a freckled child, the freckle gene was hiding in both parents. Reading the album across generations is how doctors guess whether a disease will show up in the next baby.
Several patterns of inheritance do not follow Mendel's simple rules. Polygenic inheritance occurs when many genes each add a small amount to a trait, producing continuous variation; human height and skin color are polygenic, which is why people come in a smooth range of sizes and shades rather than two or three distinct classes. Pleiotropy occurs when one gene influences many traits; sickle-cell disease, for example, arises from one altered gene and affects blood cells, pain, and organ function at once. Epistasis occurs when one gene masks the effect of another, as in Labrador retrievers, where a dog with two copies of the recessive e allele is yellow no matter what its coat-color allele at the B gene says. Cytoplasmic inheritance passes genes through mitochondria, which come only from the egg, so such traits are inherited from the mother alone. Genomic imprinting silences one parental copy of certain genes, so whether the copy came from the mother or the father can change the outcome. Environment also shapes phenotype, since a single genotype can develop differently in different conditions.
A smoothie's taste does not come from one ingredient alone. Many fruits and spices each add a little flavor, and the final taste depends on how much of each goes in. A trait like height works the same way, with many genes adding small amounts of influence. That is why such traits come in a smooth range of sizes instead of two clear groups.
Genes that sit on the same chromosome are linked and tend to travel together into the same gamete, which is why dihybrid crosses for linked genes fail to give the classic 9:3:3:1 ratio. During meiosis, crossing over can swap segments between homologous chromosomes, producing recombinant chromosomes with new combinations of alleles. The more often two linked genes recombine, the farther apart they sit on the chromosome.
Geneticists measure gene distance with the recombination frequency: divide the number of recombinant offspring by the total number of offspring and multiply by 100 percent. One percent recombination equals one map unit, also called a centimorgan. In Drosophila fruit flies, Thomas Hunt Morgan's group first demonstrated linkage with genes for body color and wing shape, and Alfred Sturtevant used recombination frequencies to draw the first chromosome map. Recombination frequencies never exceed 50 percent, the value seen for genes on different chromosomes, so a frequency below 50 percent is the signature of linkage.
Common Mistake: Confusing linked genes with sex-linked genes. Linked simply means that two genes share a chromosome, while sex-linked means that a gene sits on a sex chromosome. A gene can be linked and autosomal at the same time.
Genes on the same chromosome are like seats on the same train car. When the cars get rearranged, seats next to each other almost always stay together, while seats far apart are separated much more often. By counting how often two genes get split apart, scientists can tell how far apart the genes sit. That is exactly how geneticists draw gene maps.
High-Yield:
- A monohybrid cross gives a 3:1 F2 ratio, and a dihybrid cross gives 9:3:3:1 when the genes are unlinked.
- Incomplete dominance and codominance both give an F2 ratio of 1:2:1 at the level of phenotype.
- Sons get their X chromosome from their mother, so X-linked recessive conditions run through carrier mothers.
- Recombination frequency in percent equals map units, and values below 50 percent reveal linked genes.
- In a pedigree, unaffected parents with an affected child prove that the trait is recessive.
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