Level 2 — RecallMendelian Genetics

Mendelian Genetics

⏱ 30 minutes40 marksprintable — key stays hidden on paper

Level 2 (Recall & Standard Problems)

Time limit: 30 minutes Total marks: 40

Q1. Define the following key terms (1 mark each): (4 marks) (a) Gene (b) Allele (c) Genotype (d) Phenotype

Q2. Distinguish between a dominant allele and a recessive allele. Give one example using pea plant height. (3 marks)

Q3. A plant has the genotype $Tt$ for height and $Rr$ for seed shape. (a) Is it homozygous or heterozygous for each gene? (2 marks) (b) State the difference between homozygous and heterozygous. (2 marks)

Q4. State Mendel's Law of Segregation. (3 marks)

Q5. State Mendel's Law of Independent Assortment, and state one condition under which it applies. (3 marks)

Q6. In pea plants, purple flower colour ( $P$ ) is dominant over white ( $p$ ). Cross two heterozygous plants ( $Pp \times Pp$ ). (a) Draw the Punnett square. (2 marks) (b) State the genotypic ratio. (2 marks) (c) State the phenotypic ratio. (1 mark)

Q7. A dihybrid cross is carried out: $RrYy \times RrYy$ (round $R$ dominant over wrinkled $r$ ; yellow $Y$ dominant over green $y$ ). (a) State the phenotypic ratio expected in the F2 generation. (2 marks) (b) What fraction of offspring will be round and green? (2 marks)

Q8. A plant showing the dominant tall phenotype could be $TT$ or $Tt$ . (a) Describe how a test cross determines its genotype. (2 marks) (b) If the test cross produces some short offspring, what is the genotype of the tall parent? Explain. (2 marks)

Q9. In a cross $Aa \times Aa$ : (a) Using the product rule, what is the probability that an offspring is $aa$ ? (2 marks) (b) Using the sum rule, what is the probability that an offspring shows the dominant phenotype? (2 marks) (c) What is the probability that two offspring are both $aa$ ? (1 mark)

Q10. The pedigree below shows a recessive trait. Squares = males, circles = females; shaded = affected.

   I:   □——●        (unaffected male × affected female)
          |
   II:  ●   □        (both unaffected)

(a) If the trait is autosomal recessive, what is the genotype of the affected individual (I, female)? (1 mark) (b) The unaffected male (I) must carry at least one recessive allele if II includes an affected child. State his possible genotypes if II are all unaffected. (2 marks)

Answer keyMark scheme & solutions

Q1. (4 marks — 1 each)

(a) Gene: a unit of heredity; a segment of DNA that codes for a particular characteristic/protein. (defines heritable unit)
(b) Allele: an alternative form/version of a gene at a given locus. (distinguishes variant of gene)
(c) Genotype: the genetic (allelic) makeup of an organism (e.g. $Tt$ ). (genetic constitution)
(d) Phenotype: the observable physical/biochemical characteristics resulting from genotype + environment. (expressed trait)

Q2. (3 marks)

Dominant allele: expressed in the phenotype whenever present (in homozygous or heterozygous state) (1)
Recessive allele: only expressed when homozygous (two copies present); masked by dominant allele in heterozygotes (1)
Example: $T$ (tall) dominant, $t$ (short) recessive; $Tt$ is tall, only $tt$ is short (1)

Q3. (4 marks)

(a) $Tt$ = heterozygous; $Rr$ = heterozygous (both genes heterozygous) — 2 marks (two different alleles at each locus)
(b) Homozygous = both alleles identical ( $TT$ or $tt$ ); Heterozygous = two different alleles ( $Tt$ ) — 2 marks

Q4. (3 marks) Law of Segregation:

Each organism carries two alleles for each trait (1)
These alleles separate (segregate) during gamete formation/meiosis (1)
so each gamete receives only one allele of each gene (1)

Q5. (3 marks) Law of Independent Assortment:

Alleles of different genes segregate/assort independently of one another during gamete formation (2)
Condition: applies when the genes are on different chromosomes (or far apart/unlinked) (1)

Q6. (5 marks)

(a) Punnett square (2 marks):

	$P$	$p$
$P$	$PP$	$Pp$
$p$	$Pp$	$pp$

(b) Genotypic ratio = $1\,PP : 2\,Pp : 1\,pp$ (2 marks)
(c) Phenotypic ratio = $3$ purple : $1$ white (1 mark)

Q7. (4 marks)

(a) Phenotypic ratio = $9 : 3 : 3 : 1$ (round yellow : round green : wrinkled yellow : wrinkled green) (2 marks)
(b) Round and green: round $= \tfrac{3}{4}$ , green $= \tfrac{1}{4}$ ; product $= \tfrac{3}{4}\times\tfrac{1}{4} = \tfrac{3}{16}$ (2 marks)

Q8. (4 marks)

(a) Cross the unknown dominant plant with a homozygous recessive ( $tt$ ). Observe offspring phenotypes (2 marks) (recessive parent reveals hidden allele)
(b) Genotype is $Tt$ (heterozygous). A short ( $tt$ ) offspring must receive one $t$ from each parent; since the recessive parent is $tt$ , the tall parent must also carry $t$ , so it is $Tt$ (2 marks)

Q9. (5 marks)

(a) $P(aa) = P(a\text{ from parent 1})\times P(a\text{ from parent 2}) = \tfrac{1}{2}\times\tfrac{1}{2} = \tfrac{1}{4}$ (2 marks)
(b) Dominant phenotype = $AA$ or $Aa$ : $\tfrac{1}{4}+\tfrac{2}{4} = \tfrac{3}{4}$ (2 marks)
(c) Both $aa$ : $\tfrac{1}{4}\times\tfrac{1}{4} = \tfrac{1}{16}$ (1 mark)

Q10. (3 marks)

(a) Affected female = homozygous recessive $aa$ (1 mark)
(b) Unaffected male is $A\_$ ; since offspring in II are unaffected, he could be $AA$ or $Aa$ (both possible) (2 marks) (no affected child forces only that he is not aa; either $AA$ or $Aa$ works)

[
  {"claim":"Round-green fraction in dihybrid cross = 3/16","code":"result = (Rational(3,4)*Rational(1,4)) == Rational(3,16)"},
  {"claim":"P(aa) from Aa x Aa = 1/4","code":"result = (Rational(1,2)*Rational(1,2)) == Rational(1,4)"},
  {"claim":"P(dominant phenotype) = 3/4","code":"result = (Rational(1,4)+Rational(2,4)) == Rational(3,4)"},
  {"claim":"P(both offspring aa) = 1/16","code":"result = (Rational(1,4)*Rational(1,4)) == Rational(1,16)"}
]