A variation (different form) of the same gene.

The observable characteristics of an organism (e.g. eye colour or blood type).

The combination of alleles that control a characteristic.

A short length of DNA found on a chromosome that codes for a particular characteristic .

How many alleles do we have for each gene, and where do they come from?

Two – one inherited from the mother and one from the father .

What is a dominant allele?

An allele that only needs to be inherited from one parent for the characteristic to show up in the phenotype.

What is a recessive allele?

An allele that needs to be inherited from both parents for the characteristic to show up in the phenotype.

What does homozygous mean?

The two alleles of a gene are the same – either homozygous dominant or homozygous recessive.

What does heterozygous mean?

The two alleles of a gene are different .

Why can we not always tell an organism's genotype from its phenotype?

A phenotype caused by a dominant allele is seen in both a homozygous dominant and a heterozygous genotype.

Monohybrid inheritance

The inheritance of characteristics controlled by a single gene .

A genetic diagram showing the possible combinations of alleles that could be produced in the offspring, from which the ratio of combinations can be worked out.

In Mendel's pea plants, which allele for height is dominant and how is it shown?

The tall allele is dominant, shown as T . The short allele is recessive, shown as t .

What does the term 'pure breeding' indicate about an individual's genotype?

That the individual is homozygous for that characteristic.

A heterozygous tall plant (Tt) is crossed with a short plant. What is the short plant's genotype, and why?

tt (homozygous recessive), because it shows the recessive phenotype.

What is a family pedigree diagram used for?

To trace the pattern of inheritance of a characteristic (usually a disease) through generations of a family, and to work out the probability of inheriting it.

On a family pedigree chart, how are males and females represented?

Males by a square and females by a circle .

On a family pedigree chart, what does a horizontal line between a male and a female show?

That they have produced children (shown underneath the couple).

The chromosome pair that determines an individual's sex: XX in females and XY in males.

Which parent determines the sex of the child, and why?

The father , because only he can pass on a Y chromosome .

How is sex determined compared with most other characteristics?

Sex is determined by an entire chromosome pair , whereas most other characteristics are determined by one or a number of genes .

How can the inheritance of sex be shown?

Using a genetic diagram (Punnett square), with the X and Y chromosomes taking the place of the alleles.

Polygenic characteristic

A characteristic that is controlled by more than one gene .

Polygenic inheritance

The inheritance of characteristics that are controlled by more than one gene (poly = many).

Are most characteristics controlled by a single gene or by multiple genes?

Most are the result of multiple genes interacting, rather than a single gene.

Why is eye colour not as simple as brown being dominant to blue?

Eye colour is controlled by several genes , so there are several phenotypes beyond brown and blue – for example green and hazel .

Why is polygenic inheritance difficult to show using genetic diagrams?

Because of the wide range of combinations of features that can result.

Differences between individuals of the same species .

Variation where there are many small degrees of difference for a characteristic, usually measured on a scale (e.g. height, mass, finger length).

Discontinuous variation

Variation where there are distinct differences for a characteristic with no in-betweens (e.g. blood group, or whether you are male or female).

What are the two main causes of phenotypic variation?

Genetic – controlled entirely by genes. Environmental – caused entirely by the environment in which the organism lives.

How do meiosis and fertilisation create genetic variation?

Meiosis creates genetically different gametes carrying different alleles, and the random fusion of gametes at fertilisation gives each zygote a unique combination of alleles.

Give two examples of genetic variation in humans.

Any two of: blood group , eye colour , gender , tongue rolling , or whether ear lobes are free or fixed.

Give an example of environmental variation.

Any one of: scarring from an accident; weight gain from overeating and inactivity; the language/accent you speak; a plant growing taller in the shade to reach light.

How is human height caused by both genetic and environmental factors?

Tall parents pass on genes giving the child the genetic potential to be tall, but a poor diet (environment) means the child may not grow very well.

GCSEScienceEdexcelCombined ScienceBiologyFlashcardsGeneticsInheritance

Inheritance (Edexcel GCSE Combined Science: Biology): Flashcards

Exam code: 1SC0

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Allele

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Allele
A variation (different form) of the same gene.
Phenotype
The observable characteristics of an organism (e.g. eye colour or blood type).
Genotype
The combination of alleles that control a characteristic.
What is a gene?
A short length of DNA found on a chromosome that codes for a particular characteristic.
How many alleles do we have for each gene, and where do they come from?
Two – one inherited from the mother and one from the father.
What is a dominant allele?
An allele that only needs to be inherited from one parent for the characteristic to show up in the phenotype.
What is a recessive allele?
An allele that needs to be inherited from both parents for the characteristic to show up in the phenotype.
What does homozygous mean?
The two alleles of a gene are the same – either homozygous dominant or homozygous recessive.
What does heterozygous mean?
The two alleles of a gene are different.
Why can we not always tell an organism's genotype from its phenotype?
A phenotype caused by a dominant allele is seen in both a homozygous dominant and a heterozygous genotype.
If the two alleles of a gene are the same, the individual is ; if they are different, it is .
homozygous / heterozygous
The dominant allele is written as a letter and the recessive allele as the same letter in .
capital / lower case
Monohybrid inheritance
The inheritance of characteristics controlled by a single gene.
Punnett square
A genetic diagram showing the possible combinations of alleles that could be produced in the offspring, from which the ratio of combinations can be worked out.
In Mendel's pea plants, which allele for height is dominant and how is it shown?
The tall allele is dominant, shown as T. The short allele is recessive, shown as t.
What ratio is produced when two heterozygous (Tt) tall pea plants are crossed?
A 3 tall : 1 short ratio.
What does the term 'pure breeding' indicate about an individual's genotype?
That the individual is homozygous for that characteristic.
A heterozygous tall plant (Tt) is crossed with a short plant. What is the short plant's genotype, and why?
tt (homozygous recessive), because it shows the recessive phenotype.
From a 3:1 tall:short ratio, what is the probability of an offspring being tall, and being short?
Tall: 75%
Short: 25%
What is a family pedigree diagram used for?
To trace the pattern of inheritance of a characteristic (usually a disease) through generations of a family, and to work out the probability of inheriting it.
On a family pedigree chart, how are males and females represented?
Males by a square and females by a circle.
On a family pedigree chart, what does a horizontal line between a male and a female show?
That they have produced children (shown underneath the couple).
Crossing two heterozygous tall plants gives offspring in a ratio of tall to short.
3:1
On a pedigree chart, affected individuals are shown in and unaffected individuals in .
red / blue
Sex chromosomes
The chromosome pair that determines an individual's sex: XX in females and XY in males.
What sex chromosomes do females have?
XX.
What sex chromosomes do males have?
XY.
Which parent determines the sex of the child, and why?
The father, because only he can pass on a Y chromosome.
How is sex determined compared with most other characteristics?
Sex is determined by an entire chromosome pair, whereas most other characteristics are determined by one or a number of genes.
How can the inheritance of sex be shown?
Using a genetic diagram (Punnett square), with the X and Y chromosomes taking the place of the alleles.
Females have the sex chromosomes and males have .
XX / XY
Only the can pass on a Y chromosome, so he determines the sex of the child.
father
Polygenic characteristic
A characteristic that is controlled by more than one gene.
Polygenic inheritance
The inheritance of characteristics that are controlled by more than one gene (poly = many).
Are most characteristics controlled by a single gene or by multiple genes?
Most are the result of multiple genes interacting, rather than a single gene.
Give an example of polygenic inheritance.
Eye colour.
Why is eye colour not as simple as brown being dominant to blue?
Eye colour is controlled by several genes, so there are several phenotypes beyond brown and blue – for example green and hazel.
Why is polygenic inheritance difficult to show using genetic diagrams?
Because of the wide range of combinations of features that can result.
Characteristics that are controlled by more than one gene are described as .
polygenic
is an example of a polygenic characteristic, as it is controlled by several genes.
Eye colour
Variation
Differences between individuals of the same species.
Continuous variation
Variation where there are many small degrees of difference for a characteristic, usually measured on a scale (e.g. height, mass, finger length).
Discontinuous variation
Variation where there are distinct differences for a characteristic with no in-betweens (e.g. blood group, or whether you are male or female).
What shape of graph does continuous variation produce?
A smooth bell-shaped curve.
What shape of graph does discontinuous variation produce?
A step-like shape.
What are the two main causes of phenotypic variation?
Genetic – controlled entirely by genes.
Environmental – caused entirely by the environment in which the organism lives.
How do meiosis and fertilisation create genetic variation?
Meiosis creates genetically different gametes carrying different alleles, and the random fusion of gametes at fertilisation gives each zygote a unique combination of alleles.
Give two examples of genetic variation in humans.
Any two of: blood group, eye colour, gender, tongue rolling, or whether ear lobes are free or fixed.
Give an example of environmental variation.
Any one of: scarring from an accident; weight gain from overeating and inactivity; the language/accent you speak; a plant growing taller in the shade to reach light.
How is human height caused by both genetic and environmental factors?
Tall parents pass on genes giving the child the genetic potential to be tall, but a poor diet (environment) means the child may not grow very well.
variation gives a smooth bell-shaped curve, while variation gives a step-shaped graph.
Continuous / discontinuous
Discontinuous variation is usually caused by variation alone.
genetic
Mutation
A rare, random change in the sequence of DNA bases in a gene or a chromosome.
Why can a mutation in a gene sometimes change the protein it codes for?
Because the DNA base sequence determines the sequence of amino acids that make up a protein.
Do most mutations alter the protein a gene codes for?
No – most mutations do not alter the protein, or only alter it slightly so its appearance or function is unchanged.
What happens in an insertion mutation?
A new base is randomly inserted into the DNA sequence. This changes the amino acid coded for at that point and has a knock-on effect, changing the groups of three bases further along.
What happens in a deletion mutation?
A base is randomly deleted from the DNA sequence. This changes the amino acid coded for at that point and has a knock-on effect, changing the groups of three bases further along.
What happens in a substitution mutation, and how does it differ from an insertion or deletion?
A base is randomly swapped for a different base. It only changes the amino acid for that one group of three bases and has no knock-on effect.
How can a mutation occasionally give an individual a survival advantage?
A mutation can create a new allele and new phenotype that gives the individual a survival advantage, so it breeds more often and passes the phenotype to the next generation.
What causes sickle cell anaemia and what effect does it have?
A gene mutation changes the haemoglobin molecule, making red blood cells stiff and sickle-shaped. These cells get stuck in narrow blood vessels and block the flow of blood.
Name two things that increase the rate of mutation.
Ionising radiation (gamma rays, X-rays and ultraviolet rays).
Certain chemicals (e.g. tar in tobacco).
Why are ionising radiation and certain chemicals linked to cancer?
They increase the rate of mutation, which can cause cells to become cancerous.
An mutation has a knock-on effect on later bases, but a mutation does not.
insertion / substitution
Mutations are rare, changes in the sequence of DNA bases.
random

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