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Lecture 3

BIOLOGY 1A03 Lecture 3: THEME 5
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Department
Biology
Course
BIOLOGY 1A03
Professor
Rosa Da Silva
Semester
Winter

Description
Module 3 – Sex Chromosomes and Linkage INHERITANCE OBSERVED IN FAMILY HISTORIES • Limitations to observing human inheritance traits include few offspring and ethical issues in asking people to breed simply for the purpose of the study • For this reason, a family tree/pedigree of family history is used to segregate a specific trait of interest HUMAN GENOME COUNT 23 pairs of chromosomes 3 billion base pairs of DNA per ell 20 000 protein/RNA-coding genes Alleles linked on a single chromosome must be able to become unlinked and recombine UNIT I: THE INHERITANCE OF GENES ON THE X CHROMOSOME SEX CHROMOSOMES • Most regions of the X and Y chromosomes are largely non-homologous, meaning almost none of the genes in X have counterparts in Y, and vice versa • Recombination between these two chromosomes cannot occur • Small regions at the tips of the X and Y chromosome allow pairing and segregation SEX-LINKED GENE: located on either chromosome whose expression of the phenotype depends on the sex of the individual Y CHROMOSOME COUNT X CHROMOSOME COUNT 78 genes on the chromosome 1100 genes (most unrelated 25 proteins encoded for to sex-determination) 12.5 sex-determining proteins RECESSIVE X-LINKED INHERITANCE OF RED/GREEN COLOUR-BLINDNESS • Women who are heterozygous for the trait will not show the phenotype, but are carriers • Those who are homozygous will experience symptoms of the disease HEMIZYGOUS: any male who inherits the allele from his mother will be colour-blind for he only has one X chromosome, and therefore one locus ISHIHARA COLOUR TEST: consists of a circle containing different colour dots with an internal dot pattern forming a number that is clearly visible with normal vision RED/GREEN COLOUR-BLINDNESS PUNNET SQUARE RECESSIVE X-LINKED INHERITANCE OF HAEMOPHILIA • Results from a mutation in a gene encoding for a protein required for blood clotting HAEMOPHILIA PUNNET SQUARE: QUEEN VICTORIA UNIT II: GENETIC LINKAGE LINKED GENES ON THE X-CHROMOSOME • Mendel's second law of inheritance states that two genes sort independently during gamete formation, but this is not always the case • The homologous pair of X chromosomes in females during prophase I has 155 million bp and contains 1100 genes • There is an annotation of genes on the p arm (short) and q arm (long) DYSTRIOHIN GENE: codes for a protein needed for muscle cell development HPRT1 GENE: when mutated, leads to severe cases of acute arthritis LINKED GENES: located close together on the same chromosome, and therefore tend to be inherited together, avoiding separate segregation (i.e. Colour-blindness + Haemophilia) RECOMBINATION OF HOMOLOGOUS CHROMOSOMES • The linkage between genes can be broken in prophase I/II, resulting in individual inheritance • As a result, some offspring may inherit only one of the genes due to these recombination events ALLELE RECOMBINA
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