Genetic Crosses
Q1.
In mice, long ears are dominant over short ears. A homozygous, long-eared mouse is mated
with a short-eared mouse. Decide whether each of the following outcomes is certain, possible or
impossible.
a. No offspring will arise
b. All offspring will be long eared
c. Half of the offspring will be short-eared
d. All female offspring will be like the mother
Q2. True or False
a. If phenotype is known, the genotype will always be known
b. If two heterozygous, tall pea plants are crossed, four kinds of fertilisation are possible
c. Recessive heterozygous pea plants do not exist
d. It is possible that a tossed coin turns up heads 100 times in a row
e. Meiosis only takes place in diploid cells
Q3.
In pea plants, tall (T) is dominant over dwarf (t). A homozygous, tall plant is crossed with a
dwarf plant and two of the F1 plants are crossed. Show, by diagrams, the genotype and phenotype of
the F2 generation.
If two tall plants produce some dwarf plants, what are the genotype of the parents?
Q4.
In humans, brown eyes (B) are dominant over blue eyes (b). A brown-eyed parent and blue-eyed
parent produce four blue-eyed children over a number of years. What are the genotypes of the
parents? What are the chances that the next child will be blue-eyed too?
Q5.
Albinism is due to a recessive mutation of a single gene. Albinos are homozygous recessive (a)
for the gene and are unable to produce the pigment, melanin. This gives albinos pale skin, white hair
and pink eyes.
An albino woman marries a homozygous man and they have three children, show how all the children
are carriers for the disease.
One of the children later marries another carrier. What are the chances that they will have an albino
child?
Q6.
In snails, plain shell (S) is dominant over speckled shell (s). Two heterozygous, plain snails are
mated. State the genotypes and phenotypes of the parents. State the genotypes of any sex cells
they might produce.
Show, by diagrams, the possible genotypes and phenotypes of the F1 generation.
Assess what percentage of the F1 generation is likely to:
a. Have a plain shell
b. Have a speckled shell
c. Be homozygous
d. Be heterozygous
Q7.
In humans, the condition of brachydactyly (very short fingers) is due to the presence of a
single dominant gene.
Can two parents, with normal fingers, produce a child with brachydactyly?
Two parents with brachydactyly have a child with normal fingers what does this suggest about the
parents’ genotypes and the child’s genotype?
Q8.
In snapdragons, the gene for red flower (R) is incompletely dominant over the gene for white
flower (r) and heterozygous plants have pink flowers.
Draw a table to show all the possible genotypes and phenotypes of snapdragons.
Show, by diagrams, the genotype and phenotype of the F1 plants if a plant with red flowers is crossed
with a plant with white flowers.
If two of these F1 plants are crossed, show that the chances of obtaining plants with white flowers
is one in four.
Q9.
How many kinds of sex cell could be produced by organisms with the following genotypes?
a. AAbb
b. JjKk
c. Ccdd
d. RrSsTt
e. EEffggHHiiJJ
Q10. In potato plants, the allele for purple stems (P) is dominant to the allele for green stems (p)
and the allele for green fruit (G) is dominant to the allele for yellow fruit (g). A potato plant
heterozygous for both characteristics is crossed with a potato plant with green stems and yellow
fruit.
A) What are the genotypes of the parents?
B) State the genotype and the phenotype of the F1
C) What proportion of the offspring will have green stems and yellow fruit?
Q11. In snapdragons, the gene for red flower (R) is incompletely dominant over the gene for white
flower (r) and heterozygous plants have pink flowers. Tall (T) is dominant over dwarf (t).
Use Punnett squares to show the genotypes and phenotypes that could arise from the following
crosses:
a. A homozygous tall, red flowered plant with a dwarf, white flowered plant
b. A dwarf, pink plant with a dwarf, white plant
c. Two heterozygous tall, pink plants.
Q12. In lilies, the allele for straight stamens(S) is dominant to the allele for incurved stamens (s)
and the allele for yellow petals (Y) is dominant to the allele for white petals (y). A homozygous
straight stamen yellow flowering plant is crossed with an incurved white flowering plant.
A) State the genotype of the parents
B) State the genotype and phenotype of the F1 offspring formed
C) If two of the F1 plants were crossed, state the gametes formed, the genotype and phenotype
of the F2.
Q13. During Mendels investigations on peas, he discovered that purple flowers (P) were dominant to
white flowers (p) and green pods (G) were dominant to yellow pods (g). Mendel crossed a pea plant
homozygous for purple flower and green pod with a plant that was homozygous for white flower and
yellow pod.
A) State the genotype and phenotype of F1 plants produced.
B) Mendel went on to cross 2 of the F1 plants. Describe this cross and state the gametes formed,
genotype and phenotype of the F1 offspring produced.
C) State the ratio of phenotypes found in the F2
Q14. Mendel crossed a tall pea plant with axial flowers (TtAa) with a dwarf plant with terminal
flowers (ttaa). He obtained four types of offspring in a ratio of 1:1:1:1. Explain how this result
complied with his two laws.
Q15. In Drosophila, grey body (G) is dominant over ebony body (g) and straight wing (S) is dominant
over curled wing (s). The genes for these characteristics are linked. A grey, straight winged fly,
homozygous for both conditions, is crossed with an ebony, curled wing fly. Two of the F1 flies are
crossed. Show the genotypes and phenotypes of the F2 generation.
Q16. In sweet pea, purple flowers (P) are dominant over red flowers (p) and elongated pollen (L) is
dominant over rounded pollen (l). A plant with the genotype PPLL is crossed with a double recessive
plant (ppll). Two of the F1 plants are then crossed.
a. State the phenotypes of the parent plants.
b. Show the genotypes, phenotypes and ratio of the F2 generation:
i)
If these genes occur on separate chromosomes
ii)
If the genes are linked.
Q17. Colour blindness in humans is a sex linked condition. Draw chromosomes diagrams to illustrate
the genotypes of the following:
a. A colour blind male
b. A normal male
c. A normal female
d. A carrier for the condition
Show how it is possible for a female to have colour blindness. Can a man’s Y chromosome be inherited
from his father’s mother’s father?