Topic 16: Monohybrid Crosses

Question 1

What is albinism caused by?

Answer 1

TYR gene controlling pigment expresses recessive allele.

Question 2

What are lethal genes?

Answer 2

Result in death early in embryonic development. Do not appear thus not considered part of génotypes of a cross (becomes out of 3).

• double dose of allele is normally lethal

Question 3

What is the theoretical ratio?

Answer 3

Expected proration of génotypes and phénotypes in a large sample.

Question 4

Why might results in a small sample deviate from theoretical ratio?

Answer 4

• chance events
• sampling error
• randomness of Meiosis (indépendant assortment and crossing over) and
• fertilisation randomness

Question 5

What is a mono hybrid test cross?

Answer 5

Used where dominant phenotype is unknown as it may be homozygous or heterozygous dominant.

CROSSED WITH A HOMOZYGOUS RECESSIVE

homozygous dom x homozygous recessive
= dominant phenotype only in heterozygous genotype

heterozygous dom x homozygous recessive
= dominant or recessive phenotype with dominant in heterozygous genotypic form.