BIOL10005 Lecture Notes - Lecture 7: Mendelian Inheritance, Y Chromosome, Chromosome Segregation
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
LECTURE 7
SEX DETERMINATION & PEDIGREES
CHAPTER 9 - INHERITANCE
INHERITANCE OF COMBINATIONS OF GENES
MENDELIAN INHERITANCE IN HUMANS
•The majority of visible traits exhibit Mendelian inheritance.
•Genetic disease phenotypes can be dominant or recessive. Dominant traits tend to be found in
members of successive generations while recessive disease traits are more likely to paper without a
previous family history.
•Pedigrees or family trees represent the occurrence of a particular trait.
SEX LINKAGE AND CHROMOSOMES
SEX LINKED GENES
•Sex linked genes are genes that are inherited differently in males and females.
•Eye colour in Drosophila melanogaster is a sex linked gene. Heterozygous red and white eyes flies
crossed (female and male respectively), F1 generation all red eyed. F1 and F1 crossed and all the
female F2s were red eyed and half of the males were white eyed.
GENES ARE ON CHROMOSOMES
•Every organism has a characteristic set of chromosomes. Most eukaryotes have 2 types of
chromosomes; autosomes, which are the same in morphology (appearance) in males and females,
and sex chromosomes, which are different in morphology, are present in different numbers in
males and females and are involved in sex determination.
•(With the exception of gametes), cells in animals and plants are generally diploid cells, they
contain pairs of autosomes, one autosome of each chromosome type inherited from each parent.
•These are termed homologous chromosomes or homologs. Humans have 22 pairs of homologs
(therefore 44 autosomes) and one pair of sex chromosomes (therefore 2 sex chromosomes) in
every somatic cell.
•The separation of homologs at meiosis explains the segregation of alleles during the formation of
gametes.
•Most common sex chromosome pattern is for males to have an X chromosome and a smaller Y
chromosome and for females to have 2 X chromosomes. Both the female XX and the male Xy
behave as homologs. The Y chromosome doesn’t carry much genetic info.
•Sex linked genes are located on the X chromosome.
•Each female offspring receives either of her mother’s two X chromosomes with equal probability
but always inherits her father’s single X chromosome. Each male offspring receives either of his
mother’s two X chromosomes with equal probability but always inherits his father’s single Y
chromosome.
•Having extra chromosomes muddles with the others.
X LINKED TRAITS IN HUMANS
•Most common X linked trait in humans is red-green colour blindness. About 8% of Caucasian
males have it. Probably because the phenotype of colour blind allele is recessive and that the gene
for the blindness is on the X chromosome with no comparable gene on the Y chromosome, so
males have a higher chance than females.
Y LINKAGE AND MALE SEX DETERMINATION
•Traits influenced by the Y linked genes are passed from father to son on the Y chromosome and
are never observed in females, however this is rare because the Y chromosome carries such few
genes. The most significant Y linked mammalian gene is the sex determining gene, SRY which !
Document Summary
Mendelian inheritance in humans: the majority of visible traits exhibit mendelian inheritance, genetic disease phenotypes can be dominant or recessive. Dominant traits tend to be found in members of successive generations while recessive disease traits are more likely to paper without a previous family history: pedigrees or family trees represent the occurrence of a particular trait. Sex linked genes: sex linked genes are genes that are inherited differently in males and females, eye colour in drosophila melanogaster is a sex linked gene. Heterozygous red and white eyes flies crossed (female and male respectively), f1 generation all red eyed. Probably because the phenotype of colour blind allele is recessive and that the gene for the blindness is on the x chromosome with no comparable gene on the y chromosome, so males have a higher chance than females. The most significant y linked mammalian gene is the sex determining gene, sry which.
