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Genetics & Evolution

Genetics & Evolution - Recall a diploid zygote...

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Recall: a diploid zygote consists of the fusion of two haploid gametes: a haploid ovum from the mother and a haploid spermatozoan from the father. - Mitosis leads to the development of an adult, and the adult produces haploid gametes that divide meiotically to repeat the cell cycle once again. Every gene is located at a specific locus, but physical traits can be controlled by many different genes and therefore do not map to a single locus, but to many. Epistasis – expression of alleles for one gene is dependent on a different gene. Example: a gene for curly hair cannot be expressed if a different gene causes baldness. Human genome – 23 different chromosomes - Every cell contains two different copies (for a total of 46 chromosomes) - Two nonidentical copies of a chromosome are called homologous chromosomes . - Different versions of a gene – alleles . A person carries two copies of every gene, one on each homologous chromosome . A person can carry two different alleles. An exception to this rule is polyploidy , where individuals have more than two homologous chromosomes ( Down’s syndrome and Klinefelter syndrome ) If an individual has two different alleles at a given locus, there is a different allele on each of the two members of a homologous pair. Know heterozygote, homozygote, and phenotype - If an allele is the one expressed in the phenotype, regardless of what the second allele carried is, the expressed allele is referred to as dominant . The allele that is not expressed in the heterozygous state is recessive . - A haploid organism (like an adult fungus) cannot have recessive alleles (Why? If there is only one copy of a gene, then that is the copy which determines the phenotype) Example: CC, Cc code for curly hair, while cc codes for straight hair; there are three possible genotypes, but two phenotypes. This is called classically dominance . Incomplete dominance – a blended mix of both alleles. Codominance – two alleles are both expressed but are not blended - Example: Calico cats ; both alleles are expressed and phenotypically distinguishable Pleiotropism – alteration of a gene, resulting in unrelated aspects of the organism’s total phenotype. Polygenism – complex traits that are influenced by many different genes. - Example: Height is polygenic since it is influenced by genes for growth factors, receptors, hormones, bone deposition, muscle development, energy utilization, etc. Penetrance – likelihood that a person with a given genotype will express the expected phenotype
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It is the male gamete that determines the gender of an embryo Why? Mothers always contribute an X chromosome, regardless of whether the baby is male or female. Fathers can contribute an X or a Y chromosome, so it is the father’s contribution that determines the sex of the embryo.
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