Inheritance (OCR GCSE Combined Science A (Gateway): Biology): Flashcards
Exam code: J250
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Define gene.
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Cards in this collection (74)
Define gene.
A short length of DNA on a chromosome that codes for a particular characteristic, by coding for a protein.
Define chromosome.
A long molecule of DNA carrying many genes.
Define allele.
A variant — one of the different versions of the same gene.
Define gamete.
A sex cell, such as a sperm cell or an egg cell.
Define genotype.
The combination of alleles an organism has for a characteristic.
Define phenotype.
The observable characteristics of an organism, such as eye colour or blood group.
Define dominant allele.
An allele that only needs to be inherited from one parent for its characteristic to show in the phenotype.
Define recessive allele.
An allele that must be inherited from both parents for its characteristic to show in the phenotype.
Define homozygous.
Having two identical alleles of a gene — either two dominant or two recessive.
Define heterozygous.
Having two different alleles of a gene — one dominant and one recessive.
Define genome.
The entire genetic material of an organism.
Why does an organism have two alleles for each gene?
It inherits one from each parent, because chromosomes come in pairs.
How are dominant and recessive alleles written in genetic diagrams?
The dominant allele is a capital letter.
The recessive allele is the same letter in lower case.
Why can you not always tell a genotype from a phenotype?
A dominant characteristic shows in both the homozygous dominant and the heterozygous genotype.
What does the genome provide the instructions for?
Making all the proteins required by all the cells of the organism.
Different versions of the same gene are called ＿＿＿＿＿＿＿＿.
Different versions of the same gene are called alleles.
An organism with two identical alleles for a gene is described as ＿＿＿＿＿＿＿＿.
An organism with two identical alleles for a gene is described as homozygous.
True or False?
A recessive characteristic shows even with one recessive allele.
False.
Two recessive alleles are needed. With only one, the dominant characteristic shows instead.
True or False?
Genotype and phenotype mean the same thing.
False.
Genotype is the alleles present; phenotype is the characteristic you can observe.
Define variation.
The differences between individuals of the same species.
What two things influence the phenotype of an organism?
Its genome, and the interaction of that genome with the environment.
Define continuous variation.
Variation with many small degrees of difference, measured on a scale, such as height or weight.
Define discontinuous variation.
Variation falling into distinct categories with no in-between values, such as blood group.
Give an example of continuous variation.
Height or weight — there are many possible values in between.
Give an example of discontinuous variation.
Eye colour or blood group — you fall into one category or another.
What causes continuous variation?
Usually a combination of genes and the environment.
What causes discontinuous variation?
Usually genes alone, with no environmental influence.
What shape of graph does continuous variation give?
A smooth bell-shaped curve.
What shape of graph does discontinuous variation give?
A step-like bar chart, with separate categories.
Explain how height shows both genetic and environmental influence.
Genes give a person the potential to grow tall.
A poor diet could stop them reaching that potential, so the environment matters too.
Define mutation.
A change in the DNA base sequence, which produces a new variant of a gene.
Where do all new variants of a gene come from?
Mutations — changes to the DNA base sequence.
What are the three possible effects of a mutation on phenotype?
Most have no effect at all.
Some influence the phenotype.
A very few determine the phenotype.
Why do most mutations have no effect on the phenotype?
They either do not change the protein produced, or they change it without affecting how it works.
＿＿＿＿＿＿＿＿ variation gives a smooth bell-shaped curve when plotted.
Continuous variation gives a smooth bell-shaped curve when plotted.
New alleles are introduced into a population by ＿＿＿＿＿＿＿＿.
New alleles are introduced into a population by mutations.
True or False?
Every mutation changes the phenotype of an organism.
False.
Most mutations have no effect on the phenotype at all.
True or False?
Blood group is an example of discontinuous variation.
True.
A person is A, B, AB or O — there are no in-between groups.
Define haploid.
A cell containing half the normal number of chromosomes — one copy of each. In humans, 23.
Define diploid.
A cell containing the full number of chromosomes — two copies of each. In humans, 46.
How many chromosomes are in a human body cell and a human gamete?
A body cell has 46.
A gamete has 23.
Which type of cell division produces gametes?
Meiosis, in the reproductive organs.
Why must the chromosome number be halved to form gametes?
So that when two gametes fuse at fertilisation, the zygote has the correct diploid number and not double it.
How many cells does meiosis produce, and what are they like?
Four daughter cells.
They are haploid and genetically different from one another.
How is meiosis a source of genetic variation?
It makes new combinations of the chromosomes inherited from each parent, so every gamete is different.
Define fertilisation.
The fusion of the nuclei of two gametes, forming a zygote.
Define zygote.
The diploid cell formed when two gametes fuse at fertilisation — a fertilised egg cell.
Name the two gametes in animals.
The sperm cell and the egg cell.
Name the two gametes in flowering plants.
The pollen cell and the egg cell.
Why are offspring of sexual reproduction genetically different?
Each gamete is genetically unique, and which two fuse at fertilisation is random.
Where does half of a zygote's genetic material come from?
Half from each parent — one gamete from each.
Gametes are described as ＿＿＿＿＿＿＿＿ because they contain one copy of each chromosome.
Gametes are described as haploid because they contain one copy of each chromosome.
Meiosis involves ＿＿＿＿＿＿＿＿ divisions, which is why it produces four daughter cells.
Meiosis involves two divisions, which is why it produces four daughter cells.
True or False?
A zygote is haploid.
False.
A zygote is diploid — two haploid gametes have fused to form it.
True or False?
Meiosis produces genetically identical cells.
False.
Meiosis produces four genetically different cells. It is mitosis that produces identical ones.
Define monohybrid inheritance.
Inheritance of a characteristic controlled by a single gene.
Define Punnett square.
A genetic diagram showing the possible combinations of alleles in the offspring of a cross.
How do you set up a Punnett square?
Work out the parental genotypes and split each parent's alleles around the outside.
Fill in the four inner boxes to give the possible offspring genotypes.
What does 'pure breeding' tell you about a genotype?
The individual is homozygous for that characteristic.
Cross a homozygous tall (TT) pea plant with a homozygous short (tt).
All offspring are Tt.
All are tall, because they each inherit one dominant T allele.
Cross two heterozygous tall pea plants (Tt × Tt).
Offspring genotypes: TT, Tt, Tt, tt
Phenotype ratio: 3 tall : 1 short
In a Tt × Tt cross, what is the chance of a short offspring?
25%, or a 1 in 4 chance.
Cross a heterozygous tall plant (Tt) with a short plant (tt).
Offspring genotypes: Tt, Tt, tt, tt
Phenotype ratio: 1 tall : 1 short
Why must an organism showing a recessive phenotype be homozygous?
A recessive characteristic only shows when both alleles are recessive.
Which chromosome pair determines sex in humans?
The 23rd pair — the sex chromosomes.
What are the sex chromosomes in females and in males?
Females are XX.
Males are XY.
Use a genetic cross to show the chance of having a male child.
XX × XY gives XX, XX, XY, XY.
That is a 50% chance of a male and 50% of a female.
What is meant by multiple-gene inheritance?
Inheritance of a characteristic controlled by more than one gene, rather than by a single gene.
Are most characteristics controlled by one gene or many?
Many — most phenotypic features are the result of several genes rather than single gene inheritance.
Give an example of a polygenic characteristic.
Eye colour — several genes are involved, which is why green and hazel exist as well as brown and blue.
In a cross between two heterozygous parents, the phenotype ratio is ＿＿＿＿＿＿＿＿.
In a cross between two heterozygous parents, the phenotype ratio is 3 : 1.
Most phenotypic features are the result of ＿＿＿＿＿＿＿＿ genes rather than a single gene.
Most phenotypic features are the result of multiple genes rather than a single gene.
True or False?
A Punnett square tells you exactly what the offspring will be.
False.
It gives the probability of each outcome, not a certainty.
True or False?
The father determines the sex of a child.
True.
The mother can only pass on an X, so it is whether the father's gamete carries X or Y that decides.
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