Level 1 — RecognitionMendelian Genetics

Mendelian Genetics

⏱ 20 minutes30 marksprintable — key stays hidden on paper

Level 1: Recognition Test

Time Limit: 20 minutes Total Marks: 30

Section A — Multiple Choice (10 marks)

Choose the single best answer. 1 mark each.

The observable characteristics of an organism are its: a) genotype b) phenotype c) allele d) genome
An organism with the genotype $Tt$ is best described as: a) homozygous dominant b) homozygous recessive c) heterozygous d) hemizygous
Mendel's Law of Segregation states that: a) alleles for different genes assort independently b) the two alleles of a gene separate during gamete formation c) dominant alleles mask recessive alleles d) offspring resemble their parents
In a monohybrid cross $Tt \times Tt$ , the expected phenotypic ratio is: a) $1:1$ b) $3:1$ c) $9:3:3:1$ d) $1:2:1$
A test cross is a cross between an individual of unknown genotype and one that is: a) homozygous dominant b) heterozygous c) homozygous recessive d) a different species
In a dihybrid cross $RrYy \times RrYy$ , the expected phenotypic ratio is: a) $3:1$ b) $1:1:1:1$ c) $9:3:3:1$ d) $1:2:1$
An allele that is expressed only when present in two copies is: a) dominant b) recessive c) codominant d) heterozygous
In a pedigree chart, a filled (shaded) square usually represents: a) an unaffected female b) an affected female c) an affected male d) a carrier
The product rule is used to find the probability that: a) either of two events occurs b) two independent events occur together c) an event does not occur d) an allele is dominant
A gene is best defined as: a) an alternative form of a trait b) a unit of heredity that codes for a characteristic c) the physical appearance of an organism d) a pair of chromosomes

Section B — Matching (6 marks)

Match each term in Column X with its correct description in Column Y. 1 mark each.

Column X: (i) Allele (ii) Homozygous (iii) Genotype (iv) Independent assortment (v) Dominant allele (vi) Punnett square

Column Y:
- A. Genetic makeup of an organism
- B. Grid used to predict offspring genotypes
- C. An alternative form of a gene
- D. Genes for different traits separate independently into gametes
- E. Having two identical alleles for a gene
- F. Expressed even when only one copy is present

Section C — True/False with Justification (14 marks)

State True or False (1 mark) and give a one-line justification (1 mark). 2 marks each.

A $2:1$ phenotypic ratio is expected from crossing two heterozygotes ( $Aa \times Aa$ ).
Homozygous recessive individuals show the recessive phenotype.
The sum rule is used when calculating the probability of "either/or" outcomes.
In a cross $Aa \times aa$ , half the offspring are expected to show the dominant phenotype.
Mendel's Law of Independent Assortment applies to genes located far apart or on different chromosomes.
Two parents with a dominant phenotype can never produce an offspring with the recessive phenotype.
A dihybrid cross $RrYy \times RrYy$ produces 16 equally likely genotype combinations in the Punnett square.

Answer keyMark scheme & solutions

Section A (1 mark each)

b) phenotype — phenotype = observable traits. (1)
c) heterozygous — $Tt$ has two different alleles. (1)
b) — segregation = separation of the two alleles of a single gene into gametes. (1)
b) $3:1$ — $Tt\times Tt \to 1\,TT:2\,Tt:1\,tt$ ; dominant:recessive $=3:1$ . (1)
c) homozygous recessive — recessive tester reveals hidden alleles. (1)
c) $9:3:3:1$ — classic dihybrid ratio. (1)
b) recessive — needs two copies to be expressed. (1)
c) affected male — square = male, shaded = affected. (1)
b) — product rule = "and" of independent events. (1)
b) — a gene is a unit of heredity coding for a trait. (1)

Section B (1 mark each)

(i) Allele → C (1)
(ii) Homozygous → E (1)
(iii) Genotype → A (1)
(iv) Independent assortment → D (1)
(v) Dominant allele → F (1)
(vi) Punnett square → B (1)

Section C (2 marks each: 1 for T/F, 1 for justification)

False (1). $Aa\times Aa$ gives a $3:1$ phenotypic ratio (or $1:2:1$ genotypic), not $2:1$ . (1)
True (1). With no dominant allele present, the recessive allele is expressed as the phenotype. (1)
True (1). The sum rule adds probabilities of mutually exclusive ("either/or") events. (1)
True (1). $Aa\times aa \to \tfrac12\,Aa : \tfrac12\,aa$ , so half ( $Aa$ ) show the dominant phenotype. (1)
True (1). Genes on different chromosomes (or far apart) assort independently during meiosis. (1)
False (1). Two heterozygous dominant parents ( $Aa\times Aa$ ) can produce $aa$ offspring showing the recessive phenotype. (1)
True (1). A $4\times4$ Punnett square has $16$ boxes, each an equally likely gamete-combination outcome. (1)

[
  {"claim":"Aa x Aa gives dominant:recessive = 3:1", "code":"dom=3; rec=1; result = (dom==3 and rec==1)"},
  {"claim":"Aa x aa gives half dominant phenotype", "code":"frac_dom = Rational(1,2); result = (frac_dom == Rational(1,2))"},
  {"claim":"Dihybrid RrYy x RrYy Punnett square has 16 boxes", "code":"boxes = 4*4; result = (boxes == 16)"},
  {"claim":"Dihybrid phenotypic ratio sums to 16 parts (9+3+3+1)", "code":"total = 9+3+3+1; result = (total == 16)"}
]