Your Answer : (Not Answered)

Correct Answer : B

The other parent is B. campestris.

U's triangle shows how three diploid Brassica species combine to form three amphidiploids. B. juncea (Indian mustard, AABB, 2n = 36) arose from B. campestris (now B. rapa, AA, 2n = 20) crossed with B. nigra (BB, 2n = 16).

B. oleracea is the third diploid (CC); it is a parent of B. napus (with B. campestris) and of B. carinata (with B. nigra). B. napus and B. carinata are themselves amphidiploids, not parents of B. juncea.

Your Answer : (Not Answered)

Correct Answer : C

The ‘A’ genome came from Triticum monococcum.

Bread wheat, Triticum aestivum, is a hexaploid with the genome formula AABBDD. Its A genome comes from the diploid einkorn group: textbooks name T. monococcum, while modern studies point to its close relative T. urartu. Either way, among the options it is T. monococcum.

Aegilops tauschii, also called Ae. squarrosa (two names for one species), contributed the D genome. Ae. speltoides is closest to the B genome donor.

Your Answer : (Not Answered)

Correct Answer : C

Chromosomal elimination is a strategy to develop haploids.

In the bulbosum method, barley (Hordeum vulgare) is crossed with Hordeum bulbosum. After fertilisation the bulbosum chromosomes are progressively eliminated from the embryo, leaving a haploid with only the barley set; the embryo is rescued by culture. The wheat x maize cross works the same way. Doubling the haploid gives a fully homozygous line in one step.

Polyploids, amphidiploids and autopolyploids involve adding chromosome sets, the opposite of elimination.

Your Answer : (Not Answered)

Correct Answer : C

Down's syndrome is caused by an aberration in chromosome 21.

Most cases are trisomy 21, three copies of chromosome 21, usually from non-disjunction in the mother's meiosis; a few arise from a Robertsonian translocation carrying extra chromosome 21 material, or from mosaicism. The features include intellectual disability, a flat face and upward-slanting eyes, a single palmar crease and heart defects.

No viable syndrome involves trisomy of chromosome 2. Chromosome 20 is not the cause, and X-chromosome aneuploidies give Turner (XO) or Klinefelter (XXY) syndrome.

Your Answer : (Not Answered)

Correct Answer : D

The best parents for hybrid varieties are Inbred lines.

Hybrid varieties, as in maize, are the F₁ of a cross between two inbred lines. Repeated selfing makes each line homozygous and uniform, so their F₁ is uniform too, and lines with good combining ability are chosen to give the greatest heterosis. The same cross can be repeated each season to produce seed.

Isogenic and near-isogenic lines differ from each other at only one or a few genes, so crossing them gives almost no heterosis. Pure lines of self-pollinated crops are generally released as varieties in their own right rather than used as hybrid parents.

Your Answer : (Not Answered)

Correct Answer : C

The disorder is Phenylketonuria.

In phenylketonuria the enzyme phenylalanine hydroxylase is deficient, so phenylalanine is not converted to tyrosine. Tyrosine is the starting material for melanin, and phenylalanine also inhibits melanin synthesis, so untreated patients often have fairer skin and hair than their siblings, along with intellectual disability.

Alkaptonuria causes urine that darkens on standing and dark pigment in cartilage. Tay-Sachs disease is a ganglioside storage disorder affecting the nervous system, and Cori's disease is a glycogen storage disease caused by deficiency of the debranching enzyme.

Your Answer : (Not Answered)

Correct Answer : C

Both are X-linked recessive inheritance.

The genes for clotting factor VIII (haemophilia A) and for the red and green opsins (red-green colour blindness) lie on the X chromosome, and the disease alleles are recessive. A male, with one X, shows the trait whenever he inherits the allele; a female is usually an unaffected carrier. So these conditions are far more common in men and pass from carrier mothers to sons.

They are not autosomal, since their genes are on the X, and they are not Y-linked; the Y carries very few genes and Y-linked traits would pass only from father to son. Neither is dominant.

Your Answer : (Not Answered)

Correct Answer : B

The frequency of AABB offspring is 1/16.

The two genes assort independently, so treat each separately. From Aa x Aa the chance of AA is 1/4, and from Bb x Bb the chance of BB is 1/4:

\[ \begin{aligned} P(\mathrm{AABB}) &= \frac{1}{4} \times \frac{1}{4} \\ &= \frac{1}{16} \end{aligned} \]

This is the one homozygous dominant square in the 16-square dihybrid Punnett square. 3/16 is the share of one of the single-dominant phenotype classes (A_bb or aaB_) in the 9:3:3:1 ratio, 1/4 is the probability for one gene alone, and 1/64 would apply to a trihybrid cross.

Your Answer : (Not Answered)

Correct Answer : B

The answer is that all sons will have hypertrichosis but none of the daughters.

A Y-linked (holandric) gene is carried only on the Y chromosome, which passes from father to every son and to no daughter. An affected father therefore passes the trait to all his sons, and his daughters, who receive his X chromosome, are unaffected. The mother, having no Y chromosome, contributes nothing to the trait.

Daughters cannot be affected, which rules out the first option. 'Half the sons' would fit an X-linked gene from a carrier mother, not a Y-linked gene from the father, and 'none of the children' ignores that every son inherits his father's Y.

Your Answer : (Not Answered)

Correct Answer : A

The triplet repeat disorder is Huntington's disease.

Huntington's disease is caused by expansion of a CAG repeat in the HTT gene: more than about 36 repeats give an expanded polyglutamine tract in the huntingtin protein, which damages neurons. Like other repeat disorders it shows anticipation, with longer repeats and earlier onset in later generations.

Cystic fibrosis is usually caused by a three-base deletion (ΔF508) in CFTR, not a repeat expansion. Xeroderma pigmentosum is a defect of nucleotide excision repair, and phenylketonuria results from point mutations in the phenylalanine hydroxylase gene.

Your Answer : (Not Answered)

Correct Answer : C

A 2 : 1 ratio is due to a lethal allele.

Crossing two heterozygotes normally gives 1 AA : 2 Aa : 1 aa. If the allele is lethal when homozygous, one class dies before it can be counted, and the survivors appear in a 2 : 1 ratio. The classic case is the yellow coat allele A^Y in mice (Cuenot): yellow x yellow gives 2 yellow : 1 non-yellow, because A^YA^Y embryos die.

A plain dominant allele gives 3 : 1, a recessive one also 3 : 1, and a co-dominant (or incompletely dominant) one 1 : 2 : 1. Only lethality removes a class and leaves 2 : 1.

Your Answer : (Not Answered)

Correct Answer : A

The difference is that in Drosophila the ratio of X chromosomes to autosomes decides sex, whereas in humans the Y chromosome determines maleness.

In Drosophila an X:A ratio of 1 (XX with two sets of autosomes) gives a female and a ratio of 0.5 gives a male, so an XO fly is a (sterile) male and an XXY fly is a female; the Y is needed only for male fertility. In humans the SRY gene on the Y chromosome switches on testis development, so XXY (Klinefelter) individuals are male and XO (Turner) individuals are female.

The other options reverse the two systems: a mere pair of X chromosomes does not decide sex in the fly, the Y does not determine maleness in Drosophila, and a single X without a Y gives a female, not a male, in humans.

Your Answer : (Not Answered)

Correct Answer : B

The phenomenon is non-disjunction.

Bridges' genic balance theory says sex in Drosophila depends on the balance between X chromosomes and sets of autosomes. Non-disjunction, the failure of chromosomes to separate at meiosis, upsets that balance by producing gametes with extra or missing X chromosomes. The resulting flies, such as XXY females, XO males, and the intersexes and metafemales from triploid lines, change the X:A ratio and the sex that follows; studying them is how Bridges worked the theory out.

Linkage and crossing over rearrange alleles but do not change chromosome numbers, and sex in Drosophila is genetic, not environmental.

Your Answer : (Not Answered)

Correct Answer : B

The ABO system is an example of multiple alleles.

A single gene, I, controls the ABO blood groups, and it has three alleles in the population: \(I^A\), \(I^B\) and i. Any person carries only two of them, but the existence of more than two alternative forms of one gene is what multiple allelism means. \(I^A\) and \(I^B\) are also codominant.

Epistasis and complementation involve interactions between two different genes, and polygenic inheritance involves many genes acting together on a continuous trait such as height.

Your Answer : (Not Answered)

Correct Answer : A

Haemophilia is commoner in males because it is an X-linked disease.

The genes for clotting factors VIII and IX lie on the X chromosome, and haemophilia is X-linked recessive. A male has only one X chromosome, so a single defective copy causes the disease. A female has two X chromosomes and is usually only a carrier unless both are affected, which is rare.

An autosomal disease would affect both sexes equally. A Y-linked trait would pass only from father to son, which is not haemophilia's pattern. It is a blood clotting disorder, not a disease of the reproductive system.

Your Answer : (Not Answered)

Correct Answer : C

The match is P-(iii); Q-(iv); R-(i); S-(v); T-(ii).

• Prader-Willi syndrome is usually caused by loss of the paternal copy of a region of chromosome 15 (15q11-q13).
• Down syndrome is caused by a third copy of all or part of chromosome 21.
• Cushing syndrome results from prolonged exposure to high levels of corticosteroids such as cortisol.
• Turner syndrome is a female with one X chromosome partly or wholly missing (45,X).
• Fanconi syndrome is a failure of reabsorption in the proximal renal tubule.

The other options swap the chromosomal disorders or attach the kidney or steroid descriptions to genetic syndromes.

Your Answer : (Not Answered)

Correct Answer : A

The link was first suggested by the discovery of in-born errors of metabolism in human.

Archibald Garrod, studying alkaptonuria in the early 1900s, saw that it was inherited as a Mendelian recessive trait and that patients failed to break down homogentisic acid. He proposed that the condition was an 'inborn error of metabolism' caused by the absence of a particular enzyme, the first suggestion that genes act through enzymes.

Beadle and Tatum's work on metabolic pathways in the fungus Neurospora (1941) gave the 'one gene, one enzyme' hypothesis its experimental proof, but it came decades later. Sex determination in insects and gene regulation in bacteria are not where the idea began.

Your Answer : (Not Answered)

Correct Answer : D

The mutagen that converts adenine to hypoxanthine is nitrous acid.

Nitrous acid (HNO₂) oxidatively deaminates bases with amino groups: adenine becomes hypoxanthine, cytosine becomes uracil and guanine becomes xanthine. Hypoxanthine pairs with cytosine, so an A:T pair becomes G:C after replication, a transition.

Ultraviolet light forms pyrimidine dimers between adjacent thymines or cytosines. Mitomycin C cross-links the two strands of DNA. Methyl methanesulfonate is an alkylating agent that adds methyl groups, chiefly at N7 of guanine and N3 of adenine; it does not deaminate.

Your Answer : (Not Answered)

Correct Answer : C

The true statements are P, R.

P is true: Barbara McClintock discovered transposable elements in maize, including the autonomous Activator (Ac) and the non-autonomous Dissociation (Ds). R is true: Ac is 4563 bp long and ends in 11 bp terminal inverted repeats.

Q is false: flower colour variegation in Antirrhinum (snapdragon) is caused by its own transposons, the Tam elements, not by the maize Ac and Ds. S is false the other way round: Ac encodes the transposase and mobilises Ds, which cannot move by itself.

Your Answer : (Not Answered)

Correct Answer : A

Nullisomy is the condition in which a pair of chromosomes is missing from the somatic complement, written 2n - 2. The organism lacks both members of one pair.

Monosomy (2n - 1) lacks one chromosome only, trisomy (2n + 1) has one extra, and tetrasomy (2n + 2) has an extra pair.

The term for a chromosome number 2n - 2 is nullisomic. Nullisomics are often not viable in diploids but are found in polyploids such as wheat, where other chromosomes can make up for the missing pair.