Topic 15 Flashcards
(11 cards)
Monohybrid cross
Involves observing the inheritance of one character determined by one gene and is therefore termed a monogenic trait.
When gametes are formed in a heterozygous individual, the alleles segregate so that each gamete is equally likely to contain either allele.
Only one member of each homologous pair of chromosomes is present in each gamete so only one allele of each gene is present in the gamete.
Monogenic trait
A character brought about by one gene
ABO antigens
ABO blood group system- example of a multiple allele system for a gene.
The ABO antigens consist of sugars attached to the surface of red blood cells,
The alleles code for enzymes that join together these sugars.
O- produces a non-functioning enzyme that is unable to make any changes to the basic antigen (sugar) molecule.
A and B- co dominant and expressed equally. Each produce a different functional enzyme that adds a different specific sugar to the basic sugar molecule.
The blood group A and B antigens are able to react with antibodies present in the blood from other people and must be matched for transfusion.
Sex linkage
Special case of linkage occurring when a gene is located on a sex chromosome (usually the X)
The result of this is that the character encoded by the gene is usually see only in one sex (the heterogametic sex) and occurs rarely in the homogametic sex.
In humans, recessive sex linked genes are responsible for a number of heritable disorders in males.
Women who have recessive alleles are said to be carriers.
Example: dominant allele in humans
Rickets
Determined by a dominant allele of a gene on the X chromosome.
Co-dominant
The relationship between two alleles of a gene such that the heterozygous organism shows the expression of both alleles in the phenotype.
Lethal Genes
Some genotypes can result in death early in embryonic development.
Offspring with such lethal genotypes do not develop and so do not appear among the offspring of a cross.
Outcomes of crosses
The results of punnet squares are the expected proportion of genotypes and phenotypes in a large sample. This is called the theoretical ratio.
Results in a small sample are more likely to deviate from the theoretical ideal due to chance events, sampling error and randomness of meiosis and fertilization.
Outcomes of crosses- randomness of meiosis
All the combinations of alleles are distributed to gametes with equal probability.
Any difference between the expected and theoretical value is due to chance combination of alleles in gametes which produce offspring.
Outcomes of crosses- randomness of fertilization
Fertilisation of one pair of gametes is independent of fertilisation of another pair, these are chance events.
Monohybrid test crosses
A cross between an organism with a homozygous recessive genotype and one displaying a dominant trait.
The results of this cross can indicate the genotype of the individual displaying the dominant trait, as different results would be expected in the offspring of a large sample.
