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.
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Page 22
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.
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PKU is a recessive condition. Two
heterozygous parents (Pp) have
offspring. Predict the proportion of
offspring that will have PKU
75% chance of normal phenotype
25% chance of PKU phenotype
What is the problem with single gene
crosses?
Most characteristics are controlled by
multiple alleles rather than just one
What are sex chromosomes?
Apair 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.
50% chance of female (XX)
Other than using a punnett square, how else can monohybrid inheritance be represented?
Using a family pedigre
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% chance of haemophilia (X^hY)
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
I^B I^B
What is the genotype for blood group O?
I^O I^O
A female with genotype I^B I^O and a male with
genotype I^A I^O have a child. Use a punnett square to
predict the potential phenotypes of the offspring
25% AB
25% A
25% B
25% O
