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Flashcards · AP® Biology · Unit 5

Unit 5: Heredity flashcards

38 cards on topics 5.1 to 5.5. Pick a topic or study the whole unit.

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5.1 Meiosis

What does meiosis produce?
One diploid (2n) cell becomes four haploid (n), genetically different cells. In animals these become gametes; fertilization restores 2n. DNA replicates once, before meiosis; there is no S phase between meiosis I and II.
How do homologous chromosomes differ from sister chromatids?
Homologues come one from each parent, carry the same genes in the same order, but may have different alleles. Sister chromatids are identical copies made in S phase, joined at the centromere.
What separates in anaphase I and in anaphase II?
Anaphase I: homologous chromosomes separate while sister chromatids stay joined. Anaphase II: centromere proteins break down and sister chromatids separate.
Which stage of meiosis halves the chromosome number, and why?
Anaphase I with telophase I: homologues go to different cells, so each cell gets one of each pair. Cells at the end of meiosis I are haploid even though their chromosomes still have two chromatids.
How do chromosome and chromatid numbers change in meiosis?
Start 2n. Prophase I to metaphase I: 2n chromosomes, 4n chromatids. End of meiosis I: n chromosomes, 2n chromatids. End of meiosis II: n chromosomes, n chromatids. For 2n = 16, metaphase I has 16 chromosomes and 32 chromatids.
How do chromosomes line up in metaphase I and metaphase II?
Metaphase I: homologous pairs line up at the plate. Metaphase II: chromosomes line up in single file, with the kinetochores of the two sister chromatids attached to microtubules from opposite poles.
How does meiosis compare with mitosis?
Mitosis: 1 division, 2 cells, 2n, genetically identical. Meiosis: 2 divisions, 4 cells, n, genetically different. Both use a spindle, the same stage names, and cytokinesis by furrow or cell plate.

5.2 Meiosis and Genetic Diversity

What is crossing over, and what does it do?
In prophase I, non-sister chromatids of a homologous pair swap matching segments at chiasmata. This makes recombinant chromatids with new combinations of existing alleles; it does not create new alleles.
What is independent assortment?
Each homologous pair lines up at metaphase I independently of the others, so whole chromosomes are shuffled into gametes. It happens in metaphase I, not metaphase II.
How many chromosome combinations can gametes have?
2ⁿ, ignoring crossing over. Humans (n = 23): 2²³ ≈ 8.4 million. For 2n = 12, n = 6 gives 2⁶ = 64. Do not use 2n or n².
How does random fertilization add variation?
Any gamete from one parent can fuse with any gamete from the other. For one human couple that gives 2²³ × 2²³ = 2⁴⁶ ≈ 7.0 × 10¹³ combinations, before crossing over.
What is nondisjunction and what does it cause?
Failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate, giving gametes that are not haploid. Fertilization then gives 2n + 1 (trisomy) or 2n − 1 (monosomy) zygotes.
How do gametes differ after nondisjunction in meiosis I versus meiosis II?
Meiosis I: n + 1, n + 1, n − 1, n − 1 (all abnormal). Meiosis II in one cell: n, n, n + 1, n − 1. For n = 6, meiosis II nondisjunction gives 6, 6, 7, 5.
Why can all four products of one meiosis differ genetically?
Crossing over makes sister chromatids non-identical, so the two cells of meiosis II receive different chromatids.

5.3 Mendelian Genetics

What is the law of segregation?
Each gamete gets one allele of each gene, because homologous chromosomes separate in meiosis I.
What is the law of independent assortment?
Genes on different chromosomes are sorted independently because each homologous pair lines up at metaphase I independently. Linked genes on the same chromosome are the exception.
What are the standard Mendelian cross ratios?
Aa × Aa: 1 AA : 2 Aa : 1 aa, phenotypes 3 : 1. Aa × aa: 1 : 1. AaBb × AaBb: 9 : 3 : 3 : 1. AaBb × aabb: 1 : 1 : 1 : 1. Check whether the question asks for genotypic or phenotypic ratio.
When do you multiply and when do you add probabilities in genetics?
Multiply for independent events (separate genes, separate children): P(A and B) = P(A) × P(B). Add for mutually exclusive routes to one outcome: P(Aa) = 1/4 + 1/4 = 1/2.
What is the probability that an unaffected child of two carriers is a carrier?
2/3, not 1/2. Of the unaffected outcomes (AA, Aa, Aa), two of three are carriers.
How do you carry out a chi-square test on a genetic cross?
χ² = Σ (o − e)² ÷ e using counts, df = categories − 1, compare with the critical value at p = 0.05. 290 : 110 against 3 : 1 (expected 300 : 100): χ² = 1.33, below 3.84, so fail to reject the null hypothesis.
What pedigree clues tell dominant from recessive inheritance?
Two unaffected parents with an affected child: recessive. Two affected parents with an unaffected child: dominant. A recessive trait in a daughter whose father is unaffected: autosomal, not X-linked.
Why is a test cross made with a homozygous recessive?
It gives only recessive alleles, so each offspring's phenotype reveals the allele from the unknown parent.

5.4 Non-Mendelian Genetics

How do incomplete dominance and codominance differ?
Incomplete dominance: the heterozygote is a blend (red × white → pink). Codominance: the heterozygote shows both phenotypes fully (blood group AB, IᴬIᴮ). Both give 1 : 2 : 1 phenotypes from a heterozygote cross.
How do you detect linked genes in a test cross?
Instead of 1 : 1 : 1 : 1, there are two large (parental) classes and two small (recombinant) classes. Linked genes are on the same chromosome and tend to be inherited together.
How do you calculate recombination frequency and map distance?
Recombination frequency (%) = recombinants ÷ total offspring × 100; 1% = 1 map unit. 45 recombinants among 300: 15 map units. Divide by the total, not by parental types.
What are the clues for X-linked recessive inheritance?
More affected males; never passed father to son; can skip a generation through carrier daughters. Males (XY) have one X, so one recessive allele shows. Fathers pass X to every daughter and Y to every son.
Why are Z-linked recessive traits more common in female birds?
Birds are ZW (female) and ZZ (male), so females have one Z and a single recessive allele on it is expressed.
What is pleiotropy?
One gene with several effects, such as the sickle-cell allele causing anaemia, organ damage and malaria resistance in heterozygotes. The traits always appear together, with no recombinants.
How are mitochondrial and chloroplast genes inherited?
Organelles are randomly shared out at division and usually come from the egg or ovule, so traits are typically maternal. Reciprocal crosses differ and offspring match the mother.
How do you test whether data deviate from Mendelian ratios?
Use chi-square to compare observed counts with the Mendelian prediction. If χ² exceeds the critical value, reject the Mendelian null hypothesis and look for a biological cause such as linkage or sex linkage.

5.5 Environmental Effects on Phenotype

How does the environment affect phenotype?
Environmental signals (temperature, day length, light, diet, soil chemistry, other organisms) change which genes are expressed and how much, or how well a gene product works. The DNA sequence is not changed.
What is phenotypic plasticity?
The ability of one genotype to produce different phenotypes in different environments. The responses are not inherited as such; offspring inherit the alleles, including the ability to respond.
How does temperature determine sex in many reptiles?
In most such turtles, warm incubation raises aromatase expression, converting more testosterone to oestrogen, so ovaries develop (females). Cool eggs give males.
Why do Siamese cats have dark paws?
Their tyrosinase is heat-sensitive and only makes melanin in the cooler skin of the extremities.
Why does hydrangea flower colour vary?
In acidic soil, aluminium is taken up and flowers are blue; in alkaline soil they are pink. Same genotype, different environment.
How do you show a trait difference is environmental?
Hold genotype constant with clones, inbred lines or identical twins, and control other variables. Using seedlings of different genotypes does not isolate the environment.
How do you read a reaction norm graph?
Flat line: trait not plastic for that factor. Steep line: very plastic. Parallel lines: genotypes differ by a constant amount. Crossing lines: genotype-by-environment interaction, so the "better" genotype depends on the environment.
How can diet change the phenotype of phenylketonuria?
A low-phenylalanine diet prevents brain damage in people with two recessive alleles; the genotype is unchanged.

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