Reproduction (AQA GCSE Combined Science: Trilogy: Biology): Flashcards
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Sexual reproduction
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Sexual reproduction
The process involving the fusion (joining) of two gametes to form a zygote, producing offspring that are genetically different.
It involves a mixing of genetic information, leading to variety in the offspring.
Asexual reproduction
A process involving only one parent and no fusion of gametes, so there is no mixing of genetic information.
The offspring are genetically identical to the parent and to each other (clones).
Which type of nuclear division produces genetically identical cells, and what is it used for?
Mitosis.
It is used for growth, repair of tissues, replacement of cells and asexual reproduction.
What are the gametes in animals and in flowering plants?
Animals: sperm cells and egg cells.
Flowering plants: pollen cells and egg cells.
In sexual reproduction, the mixing of genetic information leads to in the offspring.
In sexual reproduction, the mixing of genetic information leads to variation in the offspring.
Why are the offspring of asexual reproduction genetically identical?
Only one parent is involved, so there is no fusion of gametes and no mixing of genetic information.
Only mitosis is involved.
The formation of gametes for sexual reproduction involves the type of cell division called .
The formation of gametes for sexual reproduction involves the type of cell division called meiosis.
True or False?
Asexual reproduction involves meiosis.
False.
Only mitosis is involved in asexual reproduction. Meiosis is used to form gametes for sexual reproduction.
Where in the body, and by what type of division, are gametes made?
In the reproductive organs, by meiosis.
Describe how a cell divides by meiosis to form gametes.
Copies of the genetic information are made.
The cell divides twice to form four gametes, each with a single set of chromosomes.
All the gametes are genetically different from each other.
Haploid
Describes a cell that has half the normal number of chromosomes (a single set), such as a gamete.
Meiosis produces genetically different gametes.
Meiosis produces four genetically different gametes.
What happens to the chromosome number at fertilisation?
The gametes join (fuse) to restore the normal (full) number of chromosomes, forming a zygote.
What happens to the new cell (zygote) after fertilisation?
It divides by mitosis, so the number of cells increases.
As the embryo develops, the cells differentiate (become specialised).
Meiosis halves the chromosome number so that can restore the full number.
Meiosis halves the chromosome number so that fertilisation can restore the full number.
True or False?
The four gametes produced by meiosis are genetically identical.
False.
All four gametes are genetically different from each other.
Genome
The entire genetic material of an organism.
Describe the structure of DNA.
DNA is a polymer made of two strands that form a double helix.
It is contained in structures called chromosomes.
Gene
A small section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein.
DNA is a polymer made of two strands that form a double .
DNA is a polymer made of two strands that form a double helix.
Where in a cell is DNA found?
In the nucleus, contained within structures called chromosomes.
Why is understanding the human genome important?
It allows the:
search for genes linked to different diseases
understanding and treatment of inherited disorders
tracing of human migration patterns from the past.
The of an organism is its entire genetic material.
The genome of an organism is its entire genetic material.
What does each gene code for?
A particular sequence of amino acids, which forms a specific protein.
Allele
A different form (version) of a gene.
Dominant allele
An allele that is always expressed if present, even if only one copy is inherited.
Recessive allele
An allele that is only expressed if two copies are present (i.e. no dominant allele is present).
What is the difference between genotype and phenotype?
Genotype: the combination of alleles an organism has for a characteristic.
Phenotype: the observable characteristics of an organism.
What is the difference between homozygous and heterozygous?
Homozygous: the two alleles for a characteristic are the same.
Heterozygous: the two alleles for a characteristic are different.
What is monohybrid inheritance?
The inheritance of a characteristic that is controlled by a single gene, such as fur colour in mice.
In a genetic diagram, the dominant allele is written as a letter and the recessive allele as the same letter in lower case.
In a genetic diagram, the dominant allele is written as a capital letter and the recessive allele as the same letter in lower case.
What is meant by polygenic inheritance?
Inheritance of a characteristic controlled by more than one gene.
Most characteristics result from several genes interacting, rather than a single gene.
A cross between two heterozygous pea plants (Tt × Tt) gives what phenotype ratio?
A 3 : 1 ratio of the dominant to the recessive phenotype (e.g. 3 tall : 1 short).
What is a Punnett square used for?
To show the possible combinations of alleles in the offspring of a cross, so you can predict the ratio or probability of each outcome.
In a Tt × Tt cross with a 3 : 1 ratio, what is the probability that an offspring shows the recessive phenotype?
25% (1 in 4).
Only the homozygous recessive (tt) offspring show the recessive phenotype.
What is a family tree (pedigree) diagram used to show?
It shows how a characteristic or genetic disorder is inherited through several generations of a family.
It can be used to work out genotypes and the probability of offspring inheriting the characteristic.
What causes inherited disorders?
The inheritance of certain alleles that are passed from parents to offspring.
What is polydactyly, and what type of allele causes it?
A disorder causing extra fingers or toes.
It is caused by a dominant allele.
What is cystic fibrosis, and what type of allele causes it?
A disorder of cell membranes (producing thick, sticky mucus).
It is caused by a recessive allele.
A child can only have cystic fibrosis if they inherit copies of the recessive allele.
A child can only have cystic fibrosis if they inherit two copies of the recessive allele.
If both parents are carriers of cystic fibrosis, what is the chance a child is affected?
1 in 4 (25%).
Both parents are heterozygous carriers, so on average one in four offspring is homozygous recessive and affected.
Embryo screening
Analysing the genes of an embryo (e.g. during IVF, before it is implanted) to detect genetic disorders.
Give one economic, social or ethical concern about embryo screening.
For example:
An embryo (a potential life) may be destroyed if disorder-causing alleles are found.
Some people believe selecting or altering genes in this way is unnatural.
True or False?
Polydactyly is caused by a recessive allele.
False.
Polydactyly is caused by a dominant allele, so it can be inherited even if only one parent carries it.
How many pairs of chromosomes are found in an ordinary human body cell?
23 pairs of chromosomes.
What do the 23 pairs of human chromosomes control?
22 pairs control characteristics only.
1 pair carries the genes that determine sex.
In human females the two sex chromosomes are the same, written as .
In human females the two sex chromosomes are the same, written as XX.
What are the sex chromosomes in human males and females?
Females: XX (the same).
Males: XY (different).
How can the inheritance of sex be shown?
Using a genetic diagram (Punnett square), with the X and Y chromosomes written in place of the alleles.
What ratio of male to female offspring does an XX × XY cross predict?
A 1 : 1 ratio — a 50% chance of a male (XY) and a 50% chance of a female (XX).
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