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NATS 1690 (39)


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York University
Natural Science
NATS 1690
Barbara Czaban

Genetic variation produced in sexual life cycles contributes to evolution  Mutations are the original source of genetic diversity  Changes in DNA nucleotide sequences  Creating different versions of genes (alleles)  Re-shuffling of these alleles during sexual reproduction  Meiosis and fertilization  Produces the variation that results in each member of a species having its own unique combination of traits  3 mechanisms  Crossing over  Independent assortment of chromosomes  Random fertilization Heritable variations  Among individuals in a population are transmitted from parent to offspring  Initially:  Thought that “blending” occurred when gametes combined  Creating a mixture of genetic material contributed by each parent – analogous to mixing paint colors  Blending predicts that variation would eventually disappear over many generations  Freely mating populations should give rise to a uniform population of individuals Blending prediction  Everyday observations and the results of Gregor Mendel’s breeding experiments contradicted the blending prediction  Alternative model – “Particulate mechanism of inheritance”  Parents pass on discrete heritable units (genes)  Genes retain their separate identities in offspring  Genes can be sorted and passed on  Generation after generation  In undiluted form Genes and Alleles  Alleles arise by mutations in DNA  Allele is the short form of “allelomorph” = other form  Gene is a segment of DNA located in a specific site on a specific chromosome that contains information for producing a particular protein (polypeptide).  A pair of alleles, an allele is an alternative form of a gene located on a specific site of a specific chromosome. One allele is inherited from the mother, and the other from the father. Gene locus  The location of a particular gene on a chromosome  Genetic information is carried on chromosomes that are carried in the egg and sperm in equal numbers  Homologous chromosomes have alleles for same genes at specific loci  Sister chromatids of duplicated chromosomes have same alleles for each gene Trait or inherited characteristic  Since there are 2 of each kind of chromosomes (homologous pairs) in each cell  2 genes of each trait per cell  2 alleles of each gene per trait per cell  Each trait is determined by two alleles  Type of traits  Physical – height, hair color and eye color  Behavioural – corgi dog’s instinct to herd, retriever’s desire to fetch  Predisposition to a medical condition – sickle cell anemia, some cancers and cystic fibrosis Allele Combinations  Individuals who inherited the same allele from each parent are homozygous  2 same alleles  Those with different alleles of the same gene are heterozygous  2 different alleles  When the effects of an allele are detected regardless of the alternative allele, then that allele is described a
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