Reproduction & Inheritance (Edexcel A Level Biology (A) SNAB): Flashcards
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- FrontMammalian Gametes
Define gamete.
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Define gamete.
A gamete is a sex cell, such as a sperm or egg, that fuses with another during fertilisation.
Why are gametes haploid?
They are formed by meiosis and contain only one copy of each chromosome.
This means fertilisation restores the full (diploid) chromosome number.
How many chromosomes does a human gamete contain?
23 single chromosomes.
How is a sperm cell adapted for its function?
It has a flagellum to swim to the egg.
It has many mitochondria for energy, and an acrosome of digestive enzymes to penetrate the egg.
What is the function of the acrosome in a sperm cell?
It contains digestive enzymes.
These break down the protective layer around the egg so the sperm can enter.
Why do sperm cells contain many mitochondria?
To provide energy (ATP) for movement of the flagellum, so the sperm can swim to the egg.
Why is an egg cell much larger than a sperm cell?
Most of its space is filled with food stores.
These nourish the growing embryo after fertilisation.
Define zona pellucida.
The zona pellucida is the jelly-like glycoprotein layer around the egg cell that hardens after fertilisation to block other sperm.
What is the function of the zona pellucida after fertilisation?
It forms an impenetrable barrier.
This prevents other sperm nuclei from entering the egg.
Gametes are formed by the process of , so they are haploid.
Gametes are formed by the process of meiosis, so they are haploid.
True or False?
The egg cell has a flagellum to help it move to the sperm.
False.
The sperm cell has the flagellum; the egg cell does not move towards the sperm.
What is formed when a sperm and egg fuse?
A zygote — a fertilised egg with the full diploid chromosome number.
Define fertilisation.
Fertilisation is the fusion of the nuclei of a male gamete (sperm) and a female gamete (egg).
Where does fertilisation take place in a mammal?
In the oviduct.
How do sperm cells find the egg cell?
They follow a chemical trail released by the egg.
They swim up through the cervix and uterus into the oviduct.
Define the acrosome reaction.
The acrosome reaction is the release of digestive enzymes from the sperm head, which digest a path through the egg's protective layer.
What is the role of the enzymes released in the acrosome reaction?
They digest a path through the zona pellucida.
This allows the sperm to reach and pass through the egg cell membrane.
Define the cortical reaction.
The cortical reaction is the release of cortical granules from the egg, which make the zona pellucida thicken and harden to block other sperm.
Why is the cortical reaction important?
It ensures only one sperm fertilises the egg.
The hardened zona pellucida stops any further sperm entering.
What happens immediately after the sperm and egg nuclei fuse?
A zygote is formed with the full 46 chromosomes.
Half come from the father and half from the mother.
How does a zygote develop into an embryo?
It divides repeatedly by mitosis.
The new cells then become specialised to form the body tissues.
The cortical reaction causes the zona pellucida to thicken and , preventing other sperm from entering.
The cortical reaction causes the zona pellucida to thicken and harden, preventing other sperm from entering.
True or False?
The acrosome reaction occurs in the egg cell.
False.
The acrosome reaction occurs in the sperm cell; the cortical reaction occurs in the egg.
Why does fertilisation restore the diploid chromosome number?
Each haploid gamete carries 23 chromosomes.
Their fusion combines two sets, giving the zygote the full 46.
Define locus.
A locus is the specific position of a gene on a chromosome.
Why is a gene for a characteristic always found at the same locus?
Each gene occupies a specific, fixed position on a particular chromosome.
Different alleles of that gene sit at the same locus.
Define autosomal linkage.
Autosomal linkage is when two or more genes on the same autosome are inherited together, rather than assorting independently.
What is an autosome?
Any chromosome that is not a sex chromosome.
Why do linked genes not assort independently?
They are on the same chromosome.
So they stay together in their parental combination and are passed on together.
Define sex-linked gene.
A sex-linked gene is one carried on a sex chromosome, so its inheritance depends on the sex of the individual.
What are the sex chromosome combinations for males and females?
Females are XX.
Males are XY.
On which sex chromosome are most sex-linked genes found?
The X chromosome, which is longer than the Y.
Why are males more likely to show sex-linked recessive conditions?
Males have only one X chromosome, so only one copy of an X-linked gene.
A single recessive allele is expressed, as there is no second allele to mask it.
Give two examples of sex-linked recessive conditions.
Red-green colour blindness and haemophilia.
Genes on the same autosome that are inherited together are said to be .
Genes on the same autosome that are inherited together are said to be linked.
True or False?
A female can be a carrier of a sex-linked recessive condition without being affected.
True.
With two X chromosomes, a dominant allele on one can mask the recessive allele on the other.
Define meiosis.
Meiosis is nuclear division that produces four genetically different haploid gametes from one diploid parent cell.
How many divisions and daughter cells does meiosis involve?
Two divisions (meiosis I and meiosis II).
They produce four haploid daughter cells.
During which division of meiosis does the chromosome number halve?
Meiosis I, when the homologous pairs are separated.
Name the two processes in meiosis that increase genetic variation.
Crossing over and independent assortment.
Define crossing over.
Crossing over is the exchange of alleles between non-sister chromatids of homologous chromosomes during meiosis I.
How does crossing over increase genetic variation?
Sections of chromatid break and rejoin with the other chromosome.
This produces new combinations of alleles on the chromosomes.
Define chiasma.
A chiasma is a point where non-sister chromatids cross over and exchange alleles during meiosis.
Define independent assortment.
Independent assortment is the random arrangement of homologous pairs at the equator in meiosis I, giving different allele combinations in the gametes.
How does independent assortment increase genetic variation?
Each homologous pair lines up randomly, independently of the others.
This produces many different combinations of chromosomes in the gametes.
The exchange of alleles between non-sister chromatids in meiosis is called over.
The exchange of alleles between non-sister chromatids in meiosis is called crossing over.
True or False?
Meiosis produces genetically identical daughter cells.
False.
Meiosis produces genetically different cells, because of crossing over and independent assortment.
Why is producing genetically different gametes advantageous?
It increases genetic variation in the offspring.
This provides the variation that natural selection can act on.
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