Describe the work of Gregor Mendel.
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Describe the work of Gregor Mendel.
Gregor Mendel carried out breeding experiments on peas; he observed the outcome of different crosses.
Why was Mendel's work ignored by the scientific community when he presented it?
Mendel's work was largely ignored because scientists did not understand the mechanism of inheritance, i.e. they did not know about DNA and genes.
Define the term chromosome.
A chromosome is a thread-like molecule of DNA present in the nucleus of a cell.
Define the term allele.
An allele is a version of a gene, e.g. the gene that codes for eye colour may give rise to blue or brown eyes.
True or False?
Dominant alleles are always expressed in the phenotype.
True.
Dominant alleles are always expressed in an organism's phenotype when present.
True or False?
Recessive alleles require inheritance from both parents to be expressed.
True.
Recessive alleles need to be inherited from both parents for their associated characteristic to be expressed in the phenotype. If either of the alleles inherited is dominant then it will mask the recessive form.
Define the term homozygous.
A homozygous individual has two identical alleles of a gene, e.g. DD or dd.
Define the term heterozygous.
A heterozygous individual has two different alleles of a specific gene, e.g. Dd
Define the term genotype.
Genotype refers to the combination of alleles present in an organism. A genotype can be represented by letters, e.g. EE, Ee or ee
A gene is represented by the letter G/g. What is the genotype of a homozygous dominant individual?
For a gene represented by the letter G/g the genotype of a homozygous dominant individual would be GG.
Define the term phenotype.
Phenotype refers to the observable characteristics of an organism. Phenotype is influenced by genotype and the environment.
In a species of mammal black fur is coded for by the allele B and brown fur by the allele b. What is the phenotype of an individual with the genotype Bb?
In a species of mammal where allele B = black fur and allele b = brown fur, the phenotype of an individual with the genotype Bb is black fur. This is because the dominant allele B masks expression of the recessive allele b.
Define the term gamete.
A gamete is a sex cell, e.g. a sperm cell or an egg cell. Gametes are haploid cells.
Define the term zygote.
A zygote is a fertilised egg cell.
What is monohybrid inheritance?
Monohybrid inheritance refers to the inheritance of a trait that is determined by a single gene.
How are dominant and recessive alleles usually represented in a Punnett square?
Alleles in Punnett squares are represented as follows:
dominant alleles = a capital letter
recessive alleles = the same letter in lower case
If a tall pea plant with genotype TT is crossed with a short pea plant with genotype tt, what would be the possible genotypes of the offspring?
If a tall pea plant with genotype TT is crossed with a short pea plant with genotype tt, all offspring would be heterozygous with the genotype Tt:
T | T | |
t | Tt | Tt |
t | Tt | Tt |
If a tall pea plant with genotype Tt is crossed with another tall pea plant with genotype Tt, what would be the phenotype ratio of the offspring?
If a tall pea plant with genotype Tt is crossed with another tall pea plant with genotype Tt, the phenotype ratio of the offspring would be 3: 1 (3 tall and 1 short):
T | t | |
T | TT (tall) | Tt (tall) |
t | Tt (tall) | tt (short) |
In a cross between a homozygous dominant individual and a homozygous recessive individual, what is the probability of the offspring displaying a dominant phenotype?
When a homozygous dominant individual is crossed with a homozygous recessive individual there is 100 % probability that the offspring will display the dominant phenotype. This is because all offspring will be heterozygous, e.g.:
B | B | |
b | Bb | Bb |
b | Bb | Bb |
In a cross between two heterozygous parents what is the probability of offspring having a homozygous recessive genotype?
If two heterozygous individuals are crossed there is a 1/4, 0.25 or 25 % probability of offspring having a homozygous recessive genotype, e.g.:
B | b | |
B | BB | Bb |
b | Bb | bb |
When two parents that are both heterozygous for a trait have offspring, what is the predicted ratio of offspring?
The predicted ratio of offspring characteristics from two heterozygous parents is 3:1 dominant vs recessive characteristic., e.g.:
B | b | |
B | BB (dominant) | Bb (dominant) |
b | Bb (dominant) | bb (recessive) |
What is a pedigree diagram?
A pedigree diagram is a family tree used to trace the inheritance of a specific characteristic through generations of a family.
What are the sex chromosomes?
Sex chromosomes are the chromosomes that determine the biological sex of an individual; in humans they are the X and Y chromosomes.
What are the sex chromosomes in biological females?
In biological females the sex chromosomes are XX.
What are the sex chromosomes in biological males?
In biological males the sex chromosomes are XY.
True or False?
Biological sex is determined by a single gene.
False.
Biological sex is determined by an entire chromosome pair (XX for females, XY for males), not by a single gene.
True or False?
The biological sex of a child is determined by the father.
True.
The father determines the biological sex of the child because he can pass on either an X or a Y chromosome.
What is the probability of a child inheriting two X chromosomes?
The probability of a child inheriting two X chromosomes is 1/2 or 50 %.
X | Y | |
X | XX | XY |
X | XX | XY |
Define the term codominance.
Codominance occurs when both alleles are expressed in the phenotype of an individual.
True or False?
Codominance results in the expression of one allele over the other.
False.
Codominance results in both alleles being expressed equally in the phenotype.
What does it mean for a gene to have multiple alleles?
When a gene has multiple alleles this means that there are more than two alleles for that gene present in a population.
True or False?
The gene that determines ABO blood group has multiple alleles.
True.
The gene that determines ABO blood group has three alleles: IA, IB and Io.
What is the relationship between IA, IB, and Io alleles for blood group?
The alleles for blood group show the following relationship:
IA and IB are codominant
IA and IB are dominant to Io
True or False?
A person with blood type AB has the genotype IAIB.
True.
A person with blood type AB has the genotype IAIB.
What is a sex-linked characteristic? (Higher Tier Only)
A sex-linked characteristic is one where the alleles controlling the trait are found on the sex chromosomes, typically the X chromosome.
Sex linkage makes the characteristic more common in one biological sex than in the other.
True or False?
The Y chromosome typically carries as many genes as the X chromosome. (Higher Tier Only)
False.
The Y chromosome is much smaller and carries fewer genes than the X chromosome.
Why are males more likely to show sex-linked recessive conditions than females? (Higher Tier Only)
Males are more likely to show sex-linked recessive conditions than females because they only have one X chromosome, so a recessive allele on that chromosome will always be expressed.
Which biological sex can be a carrier of sex-linked traits? (Higher Tier Only)
Females can be carriers of sex-linked traits; this is because biological females have two X chromosomes, so it is possible to have one chromosomes with an allele for the trait which is masked by the healthy allele on the other X chromosome.
Biological males have only one X chromosome, so they cannot be carriers of sex-linked traits.
What is the probability that a son born to a carrier female will have colour blindness? (Higher Tier Only)
The probability that a son born to a carrier female will have colour blindness is 50 %:
XB | Y | |
XB | XBXB | XBY |
Xb | XBXb | XbY |
For any son born from the parents shown, the probability that he will have colour blindness is 50 %.
Note that the vision of the father is irrelevant here, as any son will only inherit the Y chromosome from his father.
In a cross between a male with colour vision and a female carrying the allele for colour blindness, what is the probability of producing a child who is a carrier of colour blindness? (Higher Tier Only)
In a cross between a male with colour vision and a female carrying the allele for colour blindness, there is a 25 % chance of producing a child who is a carrier:
XB | Y | |
XB | XBXB | XBY |
Xb | XBXb | XbY |
True or False?
Most characteristics are controlled by single genes.
False.
Most characteristics are controlled by multiple genes rather than single genes.
Define the term variation.
Variation is the differences between individuals.
What is genetic variation?
Genetic variation is that which occurs due to differences in the DNA of organisms.
What causes genetic variation?
Genetic variation is generated by:
mutation
sexual reproduction
What is environmental variation?
Environmental variation is that which occurs due to factors in the environment in which an organism lives.
What factors might contribute to environmental variation?
Environmental factors such as climate, diet, accidents, culture, and lifestyle contribute to environmental variation.
Define the term mutation.
Mutations are random change to the sequence of bases in DNA.
True or False?
Genetic variation in populations occurs due to mutation.
True.
Mutations introduce genetic changes, so contribute to genetic variation between individuals in a population.
True or False?
Most mutations have a harmful effect on the phenotype.
False.
Most mutations have no effect, or just a small effect, on the phenotype. This may be the result of an unaltered proteins structure, or a minor change to protein structure that does not affect protein function.
True or False?
On rare occasions a mutation may have a significant effect on phenotype.
True.
It is rare for mutations to have a significant effect on phenotype. When this occurs it can lead to harmful effects, e.g. genetic diseases.