Topic 3 - Inheritance Flashcards
What is a chromosome?
A long, coiled molecule of DNA that carries genetic information in the form of
genes
Define gene
A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
What are alleles?
Different versions of the same gene
Define genotype
An organism’s genetic composition,
describes all alleles
Define phenotype
An organism’s observable characteristics due
to interactions of the genotype and environment
(which can modify the phenotype)
Define homozygous
Having two identical alleles of a gene e.g. FF or ff
Define heterozygous
Having two different alleles of a gene e.g. Ff
What is a dominant allele?
Describes an allele that is always expressed
Represented with a capital letter e.g. F
What is a recessive allele?
An allele that is only expressed in the
absence of a dominant allele
Represented with a small letter e.g. f
What is monohybrid inheritance?
The inheritance of a single gene
Imagine if … parents who are both heterozygous for sickle cell anaemia (Aa) have a child. Draw a genetic diagram to illustrate this single gene inheritance.
AA, Aa, Aa and aa
A female who is homozygous recessive for cystic fibrosis (ff) has a child with a heterozygous male (Ff). Draw a punnett square to illustrate this single gene inheritance
Ff, Ff, ff and ff
PKU is a recessive condition. Two heterozygous parents (Pp) have offspring. Predict the proportion of offspring that will have PKU.
PP, Pp, Pp and pp
What is the problem with single gene crosses?
Most characteristics are controlled by multiple alleles rather than just one
What are sex chromosomes?
A pair of chromosomes that determine sex:
● Males have an X and a Y chromosome
● Females have two X chromosomes
Why does the inheritance of a Y chromosome mean that an embryo develops into a male?
Testes development in an embryo is stimulated by a gene present on the Y
chromosome
A couple have a child. Using a punnett square, determine the probability of having offspring that is female.
X X
Y XY XY
X XX XX
Other than using a punnett square, how else can monohybrid inheritance be
represented?
Using a family pedigree
What is a sex-linked characteristic?
A characteristic that is coded for by an allele found on a sex chromosome.
Why are the majority of genes found on the X chromosome rather than the Y chromosome?
The X chromosome is bigger than the Y
chromosome so more genes are carried on it.
Why are men more likely to show the phenotype for a recessive sex-linked trait
than women?
● Many genes are found on the X chromosome that have no
counterpart on the Y chromosome
● Women (XX) have two alleles for each sex-linked gene whereas men
(XY) often only have one allele ∴ only one recessive allele is required
to produce the recessive phenotype in males
Haemophilia is a recessive X-linked condition. A carrier female and a normal
male have a son. What is the probability of the child having haemophilia?
50%
Give an example of a characteristic that is determined by more than one allele
Blood group is determined by three
alleles: I^A, I^B, I^O
Name the four different blood groups
A, B, AB, O
What are codominant alleles?
Alleles that equally contribute to an organism’s phenotype. They are expressed to an equal extent.
Describe codominance in blood groups
● I^A and I^B are codominant
● I^A I^B
gives the blood group AB
Why does I^A I^O give blood group A?
● I^O is recessive to I^A
● I^A is dominant and is expressed giving
blood group A
What are the possible genotypes for blood group B?
I^B I^O and I^B I^B
What is the genotype for blood group O?
I^O I^O
