Inheritance (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Flashcards
Exam code: 8465
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- FrontChromosomes & Genes
Define gene.
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Cards in this collection (30)
Define gene.
A gene is a short section of DNA on a chromosome that codes for a particular sequence of amino acids to make a specific protein.
Define genome.
The genome of an organism is the entire set of genetic material — all the genes in the DNA of its body cells.
DNA is a ......... made of two strands twisted into a ......... and is found in the ......... of cells, contained in structures called chromosomes.
DNA is a polymer made of two strands twisted into a double helix and is found in the nucleus of cells, contained in structures called chromosomes.
True or False?
Human body cells contain 23 individual chromosomes.
False.
Human body cells contain 23 pairs of chromosomes — 46 chromosomes in total.
Why does sexual reproduction lead to variation in offspring?
Sexual reproduction involves the fusion of two gametes (from different parents), each carrying different genetic information. This mixing of genetic material means offspring are genetically different from each other and from both parents.
What is meiosis, and why is it needed in sexual reproduction?
Meiosis is the type of cell division used to produce gametes (sex cells). It is needed in sexual reproduction to halve the chromosome number so that when two gametes fuse at fertilisation, the normal chromosome number is restored.
True or False?
Genes control characteristics because they code for proteins that play important roles in what cells do.
True.
Genes contain the code for proteins (including enzymes, structural proteins and hormones), which determine the characteristics of an organism.
What is DNA, and where is it found in a cell?
DNA (deoxyribonucleic acid) is the molecule that carries the instructions for growth and development of all organisms. It is found in the nucleus, contained within chromosomes.
Define sex chromosomes.
Sex chromosomes are one of the 23 pairs of chromosomes in humans that carry the genes determining sex. Females have two X chromosomes (XX); males have one X and one Y (XY).
True or False?
All egg cells contain an X chromosome.
True.
All egg cells contain an X chromosome, since females are XX. The sperm cell determines the sex of the offspring by carrying either an X or a Y chromosome.
Using a Punnett square, what is the probability of producing a male offspring?
The probability of producing a male offspring is 50% (1 in 2). A Punnett square shows that the possible combinations are XX (female) and XY (male) in equal proportions, since sperm carry either X or Y.
The sex of a human is determined by the ......... chromosome inherited from the .......... An X chromosome from the sperm produces a ........., while a Y chromosome produces a male.
The sex of a human is determined by the sex chromosome inherited from the father (sperm). An X chromosome from the sperm produces a female, while a Y chromosome produces a male.
True or False?
In humans, the mother determines the sex of the offspring.
False.
The father determines the sex. All egg cells carry an X chromosome, so it is the sperm that determines sex by carrying either an X (producing a female) or a Y (producing a male).
How many pairs of chromosomes do ordinary human body cells contain in total, and which pair determines sex?
Human body cells contain 23 pairs of chromosomes. The 23rd pair are the sex chromosomes, which determine whether an individual is male (XY) or female (XX).
Define dominant allele.
A dominant allele is always expressed in the phenotype even if only one copy is present. It is written as a capital letter in genetic diagrams.
Define recessive allele.
A recessive allele is only expressed in the phenotype if two copies are present (i.e. no dominant allele is present). It is written as a lower case letter in genetic diagrams.
An organism with two identical alleles for a gene is described as ........., while one with two different alleles is described as ..........
An organism with two identical alleles for a gene is described as homozygous, while one with two different alleles is described as heterozygous.
What is a Punnett square, and what does it show?
A Punnett square is a genetic diagram showing the possible allele combinations in offspring from a cross. It can be used to determine the ratio of different genotypes and phenotypes in the offspring.
True or False?
When two heterozygous tall pea plants (Tt) are crossed, all offspring will be tall.
False.
The cross Tt × Tt produces TT, Tt, Tt, tt — a 3:1 ratio of tall to short. The tt offspring are short because they have two recessive alleles and no dominant allele to mask it.
What ratio of tall to short plants is produced when two heterozygous tall pea plants (Tt × Tt) are crossed?
The cross produces a 3:1 ratio of tall to short. The possible offspring genotypes are TT (tall), Tt (tall), Tt (tall), and tt (short).
What is the probability of an offspring being short when a heterozygous tall plant (Tt) is crossed with a homozygous recessive short plant (tt)?
The probability is 50%. The cross Tt × tt produces Tt (tall) and tt (short) in equal numbers — a 1:1 ratio.
True or False?
A homozygous recessive plant must always show the recessive phenotype.
True.
A homozygous recessive plant (e.g. tt) has two copies of the recessive allele and no dominant allele to mask it, so the recessive phenotype is always expressed.
Define phenotype.
The phenotype is the observable characteristics of an organism, determined by its genotype and the environment in which it develops.
Define genotype.
The genotype is the set of alleles an organism has that determine a particular characteristic.
Variation in a characteristic can be due to differences in the ......... inherited, the ......... in which the organism developed, or a combination of both.
Variation in a characteristic can be due to differences in the genes inherited, the environment in which the organism developed, or a combination of both.
True or False?
Blood group in humans is an example of a characteristic determined only by genes (genetic variation).
True.
Blood group is entirely determined by genes and is not affected by the environment — it is an example of genetic variation.
Give one example of a characteristic caused by genetic variation and one example caused by environmental variation.
Genetic variation: eye colour (controlled entirely by genes)
Environmental variation: body language or accent (caused by the cultural environment in which a person is raised)
Why is height an example of a characteristic determined by both genes and the environment?
Height is influenced by multiple genes (with different alleles), giving a genetic potential. However, diet and exercise (environmental factors) also affect how tall a person actually grows, so both contribute to the final phenotype.
True or False?
An individual's genes decide which characteristics they inherit, but the environment can affect how those characteristics develop.
True.
Genes set the genetic potential for a characteristic, but environmental factors (such as diet, climate and lifestyle) can influence how that potential is expressed in the final phenotype.
Give two examples of environmental factors that can cause variation in human characteristics.
Environmental factors causing variation include:
Diet — poor diet can limit height; overeating can cause weight gain
Accidents — can cause scarring
Culture and lifestyle — determine language and accent
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