Inheritance (Edexcel IGCSE Science (Double Award): Biology): Flashcards

The entire DNA of an organism is known as its genome.

Chromosomes are found in the nucleus of a cell.

False.

The number differs between species — humans have 23 pairs, dogs have 39 pairs and rice plants have 12 pairs.

True.

Each gene codes for one specific protein.

An alternative form (version) of the same gene.

Different alleles of a gene give rise to different versions of an inherited characteristic.

Two — because humans have two copies of each chromosome, and so two copies of each gene.

One is inherited from the mother and one from the father.

Because they are different versions of the same gene, they give rise to differences in inherited characteristics.

The two matching chromosomes of a pair — one inherited from each parent.

Height in pea plants — one allele gives tall plants and the other gives short plants.

Yes. The environment also affects a characteristic, so the phenotype is not decided by the alleles alone.

A gene is a section of DNA coding for a protein. An allele is one of the alternative versions of that gene.

Alternative forms of the same gene are called alleles.

Humans have two copies of each chromosome, so they have two alleles for every gene.

False.

Only two are inherited — one from each parent — even where more than two versions exist in the population.

True.

Different alleles produce different inherited characteristics in individuals of the same species.

The observable characteristics of an organism.

The combination of alleles an organism has for a particular gene — for example TT, Tt or tt.

An allele that is expressed in the phenotype even if only one copy is present.

It is written as a capital letter.

An allele that is only expressed in the phenotype if both alleles are recessive.

It is written as a lowercase letter.

Having two identical alleles for a gene — either homozygous dominant (e.g. TT) or homozygous recessive (e.g. tt).

Having two different alleles for a gene — for example Tt.

The recessive allele is masked by the dominant one, so the dominant characteristic is shown in the phenotype.

• Dominant — a capital letter

• Recessive — the same letter in lowercase

TT and Tt.

Only tt shows the recessive characteristic.

The genotype is the combination of alleles an organism has.

The phenotype is the observable characteristic that results from it.

An organism with two identical alleles for a gene is described as homozygous.

The observable characteristics of an organism make up its phenotype.

True.

With only one copy it is masked by the dominant allele.

False.

Heterozygous means two different alleles. Two identical alleles is homozygous.

Describing a characteristic that is controlled by more than one gene (poly = many).

The inheritance of characteristics that are controlled by several genes interacting, rather than a single gene.

Many. Most characteristics result from multiple genes interacting, rather than a single gene.

Because several genes each contribute, so instead of just two options there is a continuous range of possible outcomes.

• Height

• Skin colour

• Weight

It is controlled by several genes, each contributing to how tall a person becomes, giving a continuous range from very short to very tall.

Because there are many possible combinations of alleles across the several genes involved.

A characteristic controlled by more than one gene is described as polygenic.

Human height is a polygenic characteristic, giving a continuous range from very short to very tall.

False.

Most phenotypic features are the result of polygenic inheritance — several genes interacting.

False.

They typically show a much wider range of phenotypes, rather than just two options.

Many — many genes contribute to the characteristic.

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 of a cross.

Determine the parental genotypes.

Split the alleles for each parent and write them around the outside.

Fill in the middle four squares to give the possible genetic combinations in the offspring.

Half carry the T allele and half carry the t allele, because meiosis separates the pair.

All the offspring are Tt (heterozygous), so all show the tall phenotype.

3 tall : 1 short.

The possible genotypes are TT (tall), Tt (tall) and tt (short).

• 75% chance of being tall

• 25% chance of being short

tt — homozygous recessive.

Short is the recessive phenotype, so it only shows when no dominant allele is present.

1 tall : 1 short — half the offspring are Tt (tall) and half are tt (short).

One parent was heterozygous (Tt) and the other homozygous recessive (tt).

Crossing two heterozygous pea plants gives a ratio of 3:1 tall to short offspring.

In pea plants the tall allele T is dominant and the short allele t is recessive.

True.

The ratio of allele combinations lets you calculate the percentage chance of each phenotype.

A diagram used to trace the pattern of inheritance of a specific characteristic, usually a disease, through generations of a family.

The probability that someone in the family will inherit the genetic disorder.

• Males — squares

• Females — circles

By shading or colour — affected individuals are shaded one colour and unaffected another.

That they have produced children, which are shown underneath the couple.

Underneath the couple, connected to the horizontal line joining the parents.

That the condition is not restricted to one sex — it affects both males and females.

That the characteristic is passed on continuously through the generations, appearing in each one.

On a family pedigree, males are shown as squares and females as circles.

A horizontal line between a male and a female shows that they have produced children.

True.

Squares represent males and circles represent females.

True.

This happens when both parents are carriers of a recessive allele without showing the condition themselves.

• Females — XX

• Males — XY

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

The father.

The mother can only pass on an X, but the father can pass on either an X or a Y.

• An X from the mother and an X from the father ➔ female

• An X from the mother and a Y from the father ➔ male

Using a Punnett square, with the X and Y chromosomes written in place of the alleles.

50%, since the father is equally likely to pass on an X or a Y chromosome.

The Y chromosome.

A female has the sex chromosomes XX.

Because only a father can pass on a Y chromosome, he determines the sex of the child.

False.

The mother can only pass on an X chromosome, so it is the father who determines the sex.

True.

Unlike most characteristics, which depend on individual genes, sex depends on the whole X/Y pair.

The sperm cell, because it carries either an X or a Y chromosome.

Nuclear division giving rise to genetically identical cells.

Having two copies of each chromosome. Most body cells are diploid.

Just before mitosis, each chromosome copies itself exactly, forming X-shaped chromosomes.

The chromosomes line up along the centre of the cell, where cell fibres pull them apart.

The cell divides into two, and each new cell has a copy of every chromosome.

So that when the cell splits in two, each new cell still has two copies of every chromosome — it remains diploid.

• Growth

• Repair of damaged tissues

• Cloning

• Asexual reproduction

It produces new cells — for example when a zygote divides to form an embryo.

It replaces damaged or dead cells, such as skin cells and red blood cells.

Because mitosis produces offspring that are genetically identical to the parent.

The gametes — every other cell in the body is produced by mitosis of the zygote.

Mitosis produces two cells that are genetically identical to each other.

Before mitosis each chromosome copies itself exactly, forming X-shaped chromosomes.

False.

Mitosis keeps the cells diploid — each daughter cell still has two copies of every chromosome. It is meiosis that halves the number.

True.

The chromosomes line up along the centre of the cell, and cell fibres pull them apart before the cell divides.

A type of nuclear division that gives rise to cells that are genetically different, each with half the number of chromosomes.

Four haploid daughter cells, which are the gametes.

Because otherwise the zygote would have double the number of chromosomes when the gametes join at fertilisation.

The chromosomes double themselves and line up in the centre of the cell.

First division — the chromosomes pair up along the centre, then cell fibres pull the pairs apart.

Second division — the chromosomes line up along the centre of these new cells and are pulled apart again.

The result is four haploid gametes.

• It produces the gametes — sperm and egg cells, pollen grains and ova

• It increases the genetic variation of offspring

Twice, so that only one copy of each chromosome passes to each gamete.

• Number of divisions — mitosis divides once, meiosis twice

• Number of daughter cells — mitosis makes two, meiosis makes four

• Genetic outcome — mitosis makes genetically identical diploid cells, meiosis makes genetically different haploid cells

Meiosis. Mitosis produces all the other body cells.

Meiosis produces four haploid daughter cells.

Cells with half the normal number of chromosomes are described as haploid.

False.

Meiosis produces cells that are genetically different. It is mitosis that produces identical cells.

True.

Just as in mitosis, the chromosomes copy themselves and line up in the centre of the cell before the divisions begin.

Genetic variation between the gametes produced by an individual, so each gamete carries substantially different alleles.

The fact that any male gamete can fuse with any female gamete to form a zygote.

The random fusion of gametes means each zygote receives a unique combination of alleles.

• Blood group

• Eye colour

• The ability to roll the tongue

• Whether ear lobes are free or fixed

• Meiosis, which varies the alleles between gametes

• Random fertilisation, which varies which two gametes combine

Because it results from the random fusion of two gametes, each already carrying a different combination of alleles.

Ear lobes — they are either free (lobed) or attached (lobeless).

During fertilisation, any male gamete can fuse with any female gamete to form a zygote.

Meiosis creates genetic variation between the gametes an individual produces.

False.

Meiosis makes each gamete carry a substantially different set of alleles.

True.

So are eye colour, tongue-rolling ability, and whether ear lobes are free or fixed.

It grows and develops into an adult organism.

A cell containing two complete sets of chromosomes (2n).

Nearly all human body cells are diploid, with 23 pairs — 46 chromosomes.

A cell containing one complete set of chromosomes (n) — half the number in a diploid cell.

In humans that is 23 chromosomes, with no pairs.

46 — 23 pairs.

23, with no pairs.

The gametes — the female egg and the male sperm.

Because they are involved in sexual reproduction — when two haploid gametes fuse, the zygote has the correct diploid number.

• Diploid — 2n

• Haploid — n

DNA, organised into genes that code for the proteins the cell needs.

In human cells the diploid number of chromosomes is 46.

In human cells the haploid number of chromosomes is 23.

False.

A haploid human cell has 23 chromosomes with no pairs. It is a diploid cell that has 23 pairs.

True.

The only haploid cells are the gametes — the egg and the sperm.

The differences between individuals of the same species.

• Differences in genes, brought about by random fertilisation of gametes

• Environmental factors

• A combination of both

Anything outside of the organism — including climate, diet, culture, lifestyle and accidents during a lifetime.

• Scarring on the body after an accident

• Weight gain from overeating and an inactive lifestyle

• The language and accent you speak with

• A plant growing taller in the shade of a big tree, to reach more light

Genetic variation alone. It is controlled by a single gene (monogenic), not polygenic.

• Blood group

• Ability to roll the tongue

• Whether ear lobes are free or attached

A combination of genetic and environmental factors, with the genetic component often being polygenic.

Tall parents pass on genes for height, so the children have the genetic potential to be tall.

But if their diet is poor they will not grow well — so the environment also affects their height.

Genes decide which characteristics we inherit, while the environment affects how those inherited characteristics develop.

Discontinuous variation is usually caused by genetic factors alone and is controlled by a single gene.

Variation is defined as the differences between individuals of the same species.

False.

Variation can be genetic, environmental, or a combination of both.

True.

It is polygenic and also affected by the environment, such as diet, giving a continuous range of heights.

A rare, random change in the genetic material of an organism, which can be inherited.

Yes — mutations can be passed on to offspring.

Continuously, though any individual mutation is a rare, random event.

• Genetic variation, inherited from the parents

• Environmental variation, caused by the surroundings

Most variation is a combination of both.

If the mutation occurs in a gamete, it is passed to the zygote at fertilisation and so is inherited by the offspring.

In a gene or in a chromosome.

They are a source of genetic variation, which is what natural selection acts on.

A mutation is a rare, random change in the genetic material of an organism.

A mutation can make a bacterium resistant to an antibiotic, so it survives treatment.

False.

A mutation can also give an advantage — antibiotic resistance in bacteria, for example — and natural selection then makes it more common.

True.

Mutations can be inherited.

No — they are entirely random.

Charles Darwin.

That evolutionary change has occurred, and that natural selection is the process that has driven that change.

Variation — individuals in a species vary because of differences in their genes, often caused by mutation.

Increased survival — individuals with advantageous characteristics have a higher chance of surviving.

Increased reproduction — survivors are more likely to reproduce and pass on their advantageous alleles.

Increased allele frequency — over many generations the advantageous characteristics become more common in the population.

That individuals with characteristics advantageous in their environment have a higher chance of survival.

Differences in genes, which may be caused by mutations.

Because the individuals carrying them are more likely to survive and reproduce, so they pass those alleles on to more offspring.

• Snail shell colour

• Peppered moth colour

Over many generations.

Individuals with characteristics that are advantageous in their environment have a higher chance of survival.

Over many generations, advantageous alleles become more common in the population.

False.

It is the population that changes, over many generations, as advantageous alleles become more common.

True.

This variation, often caused by mutations, is what natural selection acts on.

A random mutation gives rise to a new allele coding for antibiotic resistance.

When the population meets the antibiotic, bacteria without the allele die while those with it survive.

The survivors are more likely to reproduce, passing on the resistance allele.

Over generations the frequency of the resistance allele increases, producing a resistant strain.

Natural selection.

A random mutation in the bacterial DNA.

Only with a different antibiotic — and in some cases several antibiotics are needed.

Because the antibiotic that would normally work no longer kills the bacteria, so alternative or multiple antibiotics are needed.

They die, leaving only the bacteria carrying the resistance allele.

• Variation — a random mutation gives a resistance allele

• Increased survival — resistant bacteria survive the antibiotic

• Increased reproduction — they pass the allele on

• Increased allele frequency — the allele becomes common in the population

Antibiotic resistance first arises from a random mutation in the bacterial DNA.

Over several generations the frequency of the resistance allele increases in the population.

False.

The resistance allele arises by random mutation first. The antibiotic then selects for the bacteria that already carry it.

True.

Once resistance to one antibiotic develops, a different antibiotic — or several — must be used.

Because they are the ones that survive the antibiotic and so are the ones that reproduce, passing the allele to their offspring.