chapter 16 Flashcards
Explain the terms ‘haploid’ and ‘diploid’.
● Haploid (n) - having half of a set of
homologous chromosomes e.g. gametes
● Diploid (2n) - having the full set of homologous
chromosomes e.g. somatic cells
Explain the term ‘homologous chromosomes’.
In diploid cells, the chromosomes are in pairs - they are homologous chromosomes. Each chromosome of a pair carries the same genes, but may have different alleles. Therefore, there are two loci for each gene.
Humans have 23 pairs of chromosomes, or 46 chromosomes in total.
One chromosome of each pair is from the mother (maternal) and the other is from the father (paternal).
Why must the number of chromosomes be halved before fertilisation?
Each of the gametes contains half the number of
chromosomes (23 in humans) so that when they fuse
during fertilisation the diploid number is restored (46 in humans). If this did not occur, the number of
chromosomes would double with each generation.
Name the process of cell division that gives rise to gametes.
Meiosis
Describe the events of meiosis I.
● Prophase I - centrosomes move to opposite poles of the cell; homologous chromosomes pair up. Crossing over occur here.
● Metaphase I - homologous chromosomes line up along the equator of the cell
● Anaphase I - homologous chromosomes are separated and pulled to opposite poles of the cell by spindle fibres
● Telophase I - the nuclear envelope forms around the genetic material at each pole and the cell divides (cytokinesis)
Describe the events of meiosis II.
● Prophase II - centrosomes move to opposite poles of the cell
● Metaphase II - the chromosomes align along the equator of the cell
● Anaphase II - spindles pull the sister chromatids apart to the opposite poles of the cell
● Telophase II - the nuclear envelopes reform. Four genetically different daughter cells are produced, each with the haploid number of chromosomes.
Explain how crossing over in meiosis increases genetic variation.
During prophase I, the chromatids within a homologous pair of chromosomes twist around each other. Parts of the chromatid break off and may be rejoined with the chromatid of the other homologous chromosome. The same loci are exchanged. This creates new combinations of alleles and therefore increases genetic variation.
Explain how random assortment in meiosis increases genetic variation
During metaphase I, the homologous chromosomes align randomly along the equator. This means that in the daughter cells of meiosis I, there will be a mix of
maternal and paternal chromosomes, increasing genetic variation.
Why is the random fusion of gametes important?
It increases genetic variation due to the random combination of chromosomes generated.
Define the term ‘gene’
A gene is a sequence of DNA that codes for a polypeptide (protein) or RNA.
What is meant by the term ‘locus’?
A locus is the specific position of a gene on the chromosomes.
What is an allele?
An allele is one of the different versions of a gene. With homologous chromosomes, there are two loci for each gene; these may carry different alleles, or the same.
What is meant by homozygous and heterozygous?
Homozygous - the alleles on each of the homologous
chromosomes in a pair are the same.
Heterozygous - the alleles in the homologous pair are different
State what is meant by dominant and recessive alleles.
A dominant allele is a form of a gene that will be expressed even if only one is present (heterozygous) - it can override recessive alleles.
Recessive alleles will only be expressed if they are homozygous.
Define the term ‘codominant’.
Both alleles (which are different) are fully expressed and contribute to the phenotype.
Explain what is meant by ‘linkage’.
Two genes are said to be linked if they are close to each other on the same chromosome. They are unlikely to be separated during crossing over of meiosis, and so are inherited together.
What is a test cross?
A genetic cross to determine the genotypes of the parents and offspring. An organism with a recessive genotype is crossed with an organism whose genotype is unknown, but their phenotype shows a dominant allele is present. The phenotypes of the offspring will determine whether the parent organism is heterozygous or homozygous.
What is meant by F1 and F2?
F1 is the first generation, produced from one homozygous dominant parent and one homozygous
recessive parent. The F1 generation is therefore heterozygous.
F2 is the second generation, produced from inbreeding of the F1 generation.
Define ‘phenotype’.
The observable features of an organism.
The phenotype is a result of the
interaction between the genotype and
environment.
Define ‘genotype’.
The alleles an organism possesses.
What does a monohybrid cross show?
The possible genotypes of the offspring
for one gene only.
What is autosomal linkage?
When two genes are positioned close
together on the same autosome (any
chromosome except the sex chromosomes)
they are likely to be inherited together and so
are autosomally linked.
Explain what is meant by ‘sex linkage’.
Sex-linked genes are any genes found
on the X or Y chromosomes.
Why are males at greater risk of sex-linked disorders caused by recessive alleles?
Females = XX
Males = XY
The X chromosome is much larger than the Y chromosome, so for many of the genes on the X chromosome there is no homologue on the Y
chromosome. Therefore, recessive alleles on the X chromosome will appear more frequently in the phenotype in males, because they only have one copy
of that gene.