Genetics

(a) 1 : 2 : 1 :: Tall homozygous : Tall heterozygous : Dwarf

TT x tt gives all Tt in F1; selfing Tt gives 1 TT : 2 Tt : 1 tt in F2. The question asks for genotypes, so the ratio is 1 : 2 : 1. The 3 : 1 ratio (c) is the phenotypic ratio, and (b) mislabels the classes.

(b) (ii), (iii), (iv), (i)

Dominance: only one allele expresses in a heterozygote (ii). Codominance: both alleles express fully (iii). Pleiotropy: one gene affects many characters (iv). Polygenic inheritance: many genes govern one character (i).

(c) The two genes are linked and present on the same chromosome.

In a testcross of a dihybrid, independent assortment (genes on different chromosomes) gives parental and recombinant types in equal numbers (1:1:1:1). More parental than recombinant offspring means the genes are linked and tend to be inherited together, with crossing over separating them only occasionally. Failure of chromosomes to separate would give abnormal chromosome numbers, not this ratio.

(b) Polyploidy

If the cell plate does not form at telophase, cytokinesis fails even though the chromosomes have been duplicated and separated. The cell ends up with a doubled chromosome set, i.e. more than two complete sets, which is polyploidy. Aneuploidy is a gain or loss of single chromosomes, and polyteny is the repeated replication of chromatids without separation, as in salivary gland chromosomes.

(a) 0.25

The man is X$^c$Y and the woman is XX. All daughters are carriers (X$^c$X) and all sons are normal. Grandsons through a daughter are colour blind with probability $1/2$, while grandsons through a son get the father's Y and are normal, so the overall probability is $\tfrac12\times\tfrac12 = 0.25$.

(b) T.H. Morgan

T.H. Morgan coined the term linkage for genes inherited together on the same chromosome, based on his Drosophila work. Sutton and Boveri proposed the chromosomal theory of inheritance and Mendel formulated the laws of inheritance.

(a) controls multiple traits in an individual.

A pleiotropic gene has multiple phenotypic effects, as in the single gene for phenylketonuria or sickle-cell anaemia. It is not restricted to primitive plants and has nothing to do with the Pliocene; needing another gene describes epistasis/complementation, not pleiotropy.

(c) Pod length

Mendel's seven traits were stem height, flower colour, flower position, pod shape, pod colour, seed shape and seed colour. Pod length was not used (pod shape was), while flower position, seed colour and seed shape were.

(a) both alleles independently expressed in the heterozygote

In codominance both alleles are fully expressed in the heterozygote, as with the AB blood group (IA and IB). One allele dominating the other describes complete dominance, and codominance is unrelated to linkage.

(d) Genome, chromosome, gene, nucleotide

The genome is the total genetic material, divided into chromosomes; each chromosome carries many genes, and each gene is a sequence of nucleotides. The other options misplace the gene or the genome in the hierarchy.

(a) Chargaff's rule

Chargaff's rule (A = T, G = C) applies to double-stranded DNA, but RNA is usually single-stranded and has uracil instead of thymine. RNA still shows complementary base pairing locally, has 5' phosphate and 3' hydroxyl ends, and contains heterocyclic bases.

(a) Dominant epistasis

In summer squash, a dominant allele $W$ for white colour masks the yellow ($Y$) and green ($y$) alleles, giving a $12:3:1$ ratio (white : yellow : green) in the $F_2$. This is dominant epistasis; recessive epistasis would give $9:3:4$.

(b) 50%

The man's father was colour blind, but the man got his Y from his father, so the man is normal ($X^{c+}Y$). The woman's mother was colour blind, so the woman is a carrier ($X^{c+}X^{c}$). Sons get the mother's X, so half are colour blind: $50\%$.

(b) The genes are tightly linked

Tightly linked genes show very low recombination, not $50\%$. A $50\%$ recombination frequency means independent assortment, as seen for genes on different chromosomes or far apart on the same chromosome (with multiple crossovers).

(a) Codominance

Both $I^A$ and $I^B$ alleles express in the heterozygote, so both A and B proteins are present in AB blood group. That is codominance. In incomplete dominance a blended intermediate phenotype appears.

(b) Hershey and Chase

Hershey and Chase labelled phage DNA with $^{32}$P and protein with $^{35}$S. Only the DNA entered the bacterium, which gave the final proof that DNA is the genetic material. Griffith showed transformation, and Avery, MacLeod and McCarty identified the transforming principle as DNA but did not convince everyone.

(d) The lagging strand away from the replication fork

DNA polymerase adds nucleotides only in the 5'→3' direction. The lagging strand is therefore made discontinuously as short Okazaki fragments, each synthesised away from the fork and later joined by DNA ligase. The leading strand is continuous and grows towards the fork.

(a) Down's syndrome

Down's syndrome is trisomy 21, caused by non-disjunction of an autosome. Klinefelter's (XXY) and Turner's (XO) syndromes involve sex chromosomes. Sickle cell anaemia is a single-gene point mutation, not non-disjunction.

(c) 4 genotypes ; 3 phenotypes

The cross $I^AI^B \times I^Ai$ gives $I^AI^A$, $I^Ai$, $I^AI^B$ and $I^Bi$. That is 4 genotypes. The phenotypes are blood group A ($I^AI^A$, $I^Ai$), AB and B, which is 3 phenotypes.

(b) Trichomes – Glandular or non-glandular

Mendel studied seven pea characters: stem height, flower colour, flower position, pod shape, pod colour, seed shape and seed colour. Trichome type was not among them. Tall/dwarf, green/yellow seed and inflated/constricted pod were all studied.

(a) 1856 - 1863

Mendel conducted his hybridisation experiments on garden pea for about seven years, from 1856 to 1863. He presented his work in 1865 and published it in 1866.

(c) Thalassemia is due to less synthesis of globin molecules

Thalassemia is a quantitative defect: too little of an alpha or beta globin chain is made. Sickle cell anaemia is a qualitative defect, since a point mutation (Glu to Val) produces an abnormal beta globin. So only (c) is correct.

(b) Bacterium

Meselson and Stahl showed semiconservative replication in 1958 using the bacterium E. coli grown in $^{15}$N medium and then in $^{14}$N medium, separated by CsCl density gradient centrifugation. Taylor later showed it in plants (Vicia faba), but the first proof was in a bacterium.

(a) Starch synthesis in pea : Multiple alleles

Starch synthesis in pea is the classic example of pleiotropy (one gene, the one controlling starch-branching enzyme, affecting several traits such as seed shape and starch grain structure), not multiple alleles. The other pairs are right: ABO grouping shows co-dominance ($I^A$ and $I^B$), XO sex determination occurs in grasshopper, and Morgan worked on linkage.

(a) Ribozyme - Nucleic acid

Ribozymes are catalytic RNA molecules, so they are nucleic acids. A test cross uses the $F_1$ (not $F_2$) with the recessive parent, Morgan worked on linkage (not transduction), and transformation was shown by Griffith, not Mendel.

(b) a, b and c

ABO blood groups show dominance ($I^A$ and $I^B$ over $i$), co-dominance ($I^A$ and $I^B$ both expressed in AB) and multiple alleles (three alleles $I^A$, $I^B$, $i$). They do not show incomplete dominance or polygenic inheritance.

(d) Both sons and daughters

A woman has two X chromosomes, and each of her children receives one of them, so the affected X can pass to a son or daughter with a probability of 1/2. It is not limited to one sex and is not limited to grandchildren.

(b) Deletion of GGU from 7th, 8th and 9th positions

Read in codons the mRNA is AAC AGC GGU GCU AUU. Deleting GGU (positions 7-9) removes a whole codon, so the downstream codons stay in frame. Inserting or deleting one base (options c and d), or inserting two bases (option a), shifts the frame. Only changes of a multiple of three bases keep the frame.

(d) Adenine and guanine

Purines are adenine and guanine, and both occur in DNA and RNA. Cytosine, thymine and uracil are pyrimidines. Thymine is found only in DNA and uracil only in RNA, which rules out options (b) and (c). Option (a) pairs a purine with a pyrimidine.

(b) Law of Segregation does not apply in this experiment

The $F_2$ ratio is $1:2:1$ (red : pink : white), which shows that the alleles segregate in the gametes. The law of segregation therefore applies. Dominance does not apply because the heterozygote is intermediate, and the pink colour comes from incomplete dominance. Statement (b) is the false one.

(a) In domesticated fowls, sex of progeny depends on the type of sperm rather than egg

Birds have the ZW-ZZ system, in which the female (ZW) is heterogametic. The sex of the progeny therefore depends on the egg (Z or W), not the sperm. The other statements are correct: the human Y is much shorter than X, the male fruit fly (XY) is heterogametic, and male grasshoppers are XO, so half the sperms lack a sex chromosome.

(a) A unit of distance between genes on chromosomes, representing 1% cross over.

One map unit (centimorgan) equals 1% recombination frequency between two linked genes. A larger recombination percentage means the genes are further apart. The 10%, 50% and 100% figures in the other options are wrong.

(a) Alfred Sturtevant

Sturtevant, a student of Morgan, used the frequency of recombination between linked genes as a measure of the distance between them. He used this to build the first genetic maps. Morgan found linkage, and Sutton and Boveri proposed the chromosomal theory. Mendel did not work on linkage.

(b) Morgan

Sutton and Boveri proposed the chromosomal theory of inheritance. Morgan verified it experimentally through his work on sex linkage in Drosophila. Mendel worked before chromosomes were linked to inheritance.

(a) 14

Mendel chose 14 true-breeding pea varieties, forming 7 pairs, each differing in one character with contrasting traits. The pairs were tall/dwarf, round/wrinkled seed and so on. Option (b), 8, is not the number used.

(c) Adenine pairs with thymine through two H-bonds

In DNA, A pairs with T by two hydrogen bonds and G pairs with C by three. This is why GC-rich DNA is harder to denature. Option (a) wrongly swaps the numbers, and (b) is false because A–T pairing is the rule (Chargaff).

(a) When $I^A$ and $I^B$ are present together, they express same type of sugar

This is the wrong statement. $I^A$ and $I^B$ are codominant and produce two different sugars (A and B), so the $AB$ blood group has both. The gene has three alleles ($I^A$, $I^B$, $i$), allele $i$ makes no sugar, and a diploid person carries only two of them.

(a) Sickle cell anaemia - Autosomal recessive trait, chromosome-11

Sickle cell anaemia results from a single base substitution (GAG to GUG) in the $\beta$-globin gene on chromosome 11, and is autosomal recessive. Thalassemia is autosomal, haemophilia is X-linked recessive (not Y-linked), and phenylketonuria is autosomal recessive (not dominant).

(a) 2.2 meters

Length = number of base pairs $\times$ spacing $= 6.6\times10^{9}\times0.34\times10^{-9}\,\text{m} = 2.244\,\text{m}\approx 2.2\,\text{m}$. The nearest wrong options come from rounding incorrectly.

(b) A single gene affecting multiple phenotypic expression

Pleiotropy is when one gene influences several traits, for example the gene for phenylketonuria or sickle-cell anaemia producing multiple effects. Several genes controlling one character is polygenic inheritance, and several alleles of one gene is multiple allelism.

(b) Alfred Sturtevant

Sturtevant, a student of Morgan, used recombination frequency between gene pairs as a measure of distance and mapped genes on the chromosome. Morgan discovered linkage, and Sutton and Boveri proposed the chromosomal theory of inheritance.

(a) Alfred Hershey and Martha Chase

Hershey and Chase used radioactive $^{32}$P (DNA) and $^{35}$S (protein) in bacteriophages and showed that only DNA enters the bacterium, giving unequivocal proof. Griffith showed transformation, and Avery, MacLeod and McCarty showed DNA was the transforming principle but with less decisive proof.

(b) B and E only

Klinefelter syndrome (XXY) gives overall masculine development with some feminine traits such as gynaecomastia, and the individuals are sterile. A describes Down syndrome (Langdon Down), and C and D do not describe Klinefelter syndrome (they relate to Turner and Down syndromes).