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Linked genes, sex linkage and pedigrees
Transcript of Linked genes, sex linkage and pedigrees
Linkage - is the tendency for two or more genes located on the same chromosome to be inherited together.
Gene Complex - are two or more genes that are so tightly linked that they rarely result in recombination.
what is a gene complex and a gene marker?
The % of recombination betweeen two linked genes is correlated with their physical distance apart along the length of the chromosome.
Recombinant gametes carry a combination of alleles not observed in the parents
Genes are said to be linked when the % of recombinant gametes falls below 50%
Genetic Recombination and Gene Mapping
Continuous and discontinuous variation
is the determination of the pattern of inheritance of a condition (or disease) by reference to a family tree in which the presence or absence of the condition is recorded over generations
Sex-linked inheritance is the inheritance pattern determined by genes located on the sex chromosomes.
Epidemiology is the description and analysis of the pattern of diseases in the population, the causes of these different patterns, and the use of this information to improve public health.
genes in populations
What is the likely mode of inheritance in this example?
What can you from this pedigree?
Hairy ears is one of few examples - the y chromosome has few genes, most are involved in male sex determination and fertility. If a trait is passed from father to son and never observed in females it is y-linked, meaning the gene for that trait is on the y chromosome.
Crossing over - is the exchange of chromosomal material between members of a chromosomal pair during meiosis.
When two genes A and B are on the same chromosome, crossing over may occur elsewhere on the chromosome or between the two genes. The gametes produced as a result of these two situations are very different.
What is a chiasma?
A chiasma is the point of crossing of strands of non-sister chromatids observed during the first division of meiosis.
Gene Marker - a gene that is closely linked to another gene. The closer the two loci are, the greater the linkage and more precisely a genotype at one locus can be used to predict the other.
X-linked recessive inheritance
x-linked dominant inheritance
x-linked recessive inheritance shows a pattern of transmission of the mutant phenotype from the female parent to male offspring
x-linked dominant inheritance shows a pattern of transmission of the mutant phenotype from an affected male parent to all female offspring, and from an affected heterozygous female parent to 50% of all offspring.
Pedigree charts are a way of graphically illustrating inheritance patterns over a number of generations. They are used to study the inheritance of genetic disorders.
Given sufficient data we can work out the patterns of inheritance - dominance, autosomal, sex-linked. See fig. 15.12 p344 of text - flow diagram for pedigree analysis of simple modes of inheritance
When alternative phenotypes of a particular characteristic are clearly different, this is known as discontinuous variation.
When there are many intermediate phenotypic forms of a given characteristic and the distribution of the phenotypes in a population is a bell curve, this is known as continuous variation.
Continuous variation results from the combined effects of multiple genes.
single genes are usually associated with discontinuous variation in phenotype
polygenic traits are controlled by many genes - most traits are polygenic!!!!!!!!!!!!!!!!!!!!!!!!!!
some very severe mutations are retained in the population because of heterozygote advantage eg sickle cell anaemia and thalassaemia - increases resistance to infection by malaria
why do fatal genetic diseases continue to be present in the population????????????