Unit 5: Heredity

21 terms. Say what each one means, then open the card to check.

• Meiosis

  A cell division with one round of DNA copying and two divisions. It turns one diploid cell into four haploid cells, each with one chromosome from every pair.

  Example. A fictional moss with 2n = 16 makes spores with 8 chromosomes each after meiosis.

• Homologous chromosomes

  A matched pair of chromosomes, one from each parent, with the same genes in the same order. They may carry different alleles of those genes.

  Example. A person's two copies of chromosome 7 are homologous: both carry the same genes, but one came from the mother and one from the father.

• Crossing over

  The swap of matching segments between non-sister chromatids of a homologous pair during prophase I. It makes chromatids with new combinations of alleles.

  Example. A chromatid that carried alleles A and B ends up carrying A and b after a segment is swapped with its homolog.

• Independent assortment

  Each homologous pair lines up at metaphase I without regard to the other pairs, so the maternal and paternal chromosomes of different pairs are sorted into gametes in random combinations.

  Example. With 4 chromosome pairs, one parent can make 2 x 2 x 2 x 2 = 16 kinds of gametes even before crossing over.

• Allele

  One of the different versions of a gene. A diploid cell carries two alleles of each gene on its autosomes, which may be the same or different.

  Example. A fictional beetle gene for shell shine has a shiny allele S and a dull allele s.

• Genotype and phenotype

  The genotype is the set of alleles an organism carries; the phenotype is the trait that shows. The same phenotype can come from more than one genotype.

  Example. Beetles with genotype SS and Ss both have shiny shells, so they share a phenotype but not a genotype.

• Law of segregation

  The two alleles of a gene separate from each other when gametes form, so each gamete carries just one of them. This follows from homologs separating in meiosis I.

  Example. A plant with genotype Rr makes gametes that are half R and half r.

• Test cross

  Crossing an organism that shows the dominant trait with one that is homozygous recessive. The offspring reveal whether the first organism is homozygous or heterozygous.

  Example. A shiny beetle crossed with a dull (ss) beetle gives some dull young, so the shiny parent must be Ss.

• Product rule

  The chance that two independent events both happen is the product of their separate chances. It lets you combine the results of genes that assort independently.

  Example. From Aa x Aa the chance of aa is 1/4, and from Bb x bb the chance of bb is 1/2, so the chance of aabb is 1/8.

• Chi-square test

  A test that compares observed counts with the counts a hypothesis predicts. A value larger than the critical value means the gap is too big to blame on chance alone.

  Example. Counts of 156 and 44 against an expected 150 and 50 give a chi-square of 0.24 + 0.72 = 0.96, below the critical value of 3.84 for one degree of freedom.

• Phenotypic plasticity

  The ability of one genotype to produce different phenotypes in different environments, such as different temperatures, diets or soil conditions.

  Example. Clones of a fictional water plant grow thin, ribbon-like leaves underwater and broad leaves above the surface.

• Temperature-sensitive enzyme

  An enzyme that works at some temperatures but loses its shape and stops working at others, so the trait it controls depends on body or air temperature.

  Example. In a fictional rabbit, a pigment enzyme works only below about 33 degrees C, so only the cooler ears, nose and paws turn dark.

• Incomplete dominance

  A pattern in which the heterozygote shows a phenotype in between the two homozygotes. The alleles themselves stay unchanged and can reappear in the next generation.

  Example. In a fictional snapdragon, red x white gives pink, and pink x pink gives red, pink and white in a 1 : 2 : 1 ratio.

• Codominance

  A pattern in which a heterozygote shows both alleles' traits fully and at the same time, not a blend of the two.

  Example. A person with alleles for both the A and the B marker on red blood cells has type AB blood, with both markers present.

• Multiple alleles

  A gene that has more than two alleles in a population, although each diploid individual still carries only two of them.

  Example. The human ABO gene has three common alleles, I-A, I-B and i, which give six genotypes and four blood types.

• Polygenic trait

  A trait shaped by many genes, each adding a small effect. It varies continuously, often in a bell-shaped spread, rather than in a few clear classes.

  Example. Seed weight in a fictional grass ranges smoothly from light to heavy because many genes each add a little weight.

• Sex-linked gene

  A gene on a sex chromosome, usually the X. Males (XY) have one X, so a single recessive allele on it shows in their phenotype.

  Example. A son with a recessive X-linked allele for a color vision defect got that X from his mother, since his father gave him a Y.

• Linked genes

  Genes close together on the same chromosome. They tend to be inherited together and separate only when crossing over occurs between them.

  Example. Two fly genes on one chromosome stay in their parental combinations in most offspring, and only a few young show new combinations.

• Recombination frequency

  The percent of offspring with new combinations of linked alleles. One percent counts as one map unit, and genes farther apart recombine more often, up to about 50 percent.

  Example. If 36 of 300 test-cross offspring are recombinant, the frequency is 12 percent, so the genes are about 12 map units apart.

• Nondisjunction

  The failure of homologs (in meiosis I) or sister chromatids (in meiosis II) to separate. It makes gametes with one chromosome too many or too few.

  Example. A gamete with two copies of chromosome 21 joins a normal gamete, and the zygote has three copies: trisomy 21.

• Inheritance of mitochondrial genes

  Genes in mitochondria and chloroplasts are outside the nucleus. In humans and many other species they pass to offspring mainly through the egg, so they follow the mother.

  Example. A woman with a mitochondrial gene variant can pass it to both her sons and her daughters, while an affected man does not pass it to his children.