Genetics Chapter 6 PDF

Chapter 6 Genetics & Sexuality By Reem Dakhil I. Sex-determination Systems 1. Lygaus 2. Abraxas 3. Protenor Lygaus Mode ♀ Female XX  produces only 1 gamete X  homogametic ♂ Male XY  produces 2 types of gametes X & Y  heterogametic Abraxas Mode ♀ Female ZW  produces...

Chapter 6 Genetics & Sexuality By Reem Dakhil I. Sex-determination Systems 1. Lygaus 2. Abraxas 3. Protenor Lygaus Mode ♀ Female XX  produces only 1 gamete X  homogametic ♂ Male XY  produces 2 types of gametes X & Y  heterogametic Abraxas Mode ♀ Female ZW  produces 2 types of gametes Z & W  heterogametic ♂ Male ZZ  produces only 1 type of gamete Z  homogametic Protenor Mode ♀ Female XX  produces only 1 gamete X  homogametic ♂ Male XO  produces 2 types of gametes X & no X  heterogametic I. Modes of Sex Determination Mode System Mechanism Heterogametic Sex Organisms Some grasshoppers and Protenor XX-XO Females XX Males X Male other insects Many insects, fish, Lygaus XX-XY Females XX Males XY Male amphibians, reptiles; mammals, including humans Butterflies, birds; some Abraxas ZZ-ZW Females ZW Males ZZ Female reptiles and amphibians The human male sex chromosome (Y, at the left) differs from the human female sex chromosome (X, at the right) in size and shape. Human Sex Chromosomes X & Y Chromosome Y II. Chromosome Y - Pseudo Autosomal Region (PAR): homologous with X, found at both ends  behave as autosomes - Holandric genes: non-homologous part (transferred from father to son)  represented by 1 allele. The gene SRY (Sex-determining Region of Y) determines maleness. In Abraxas mode: the non-homologous part are called hologynic genes (W) transferred from mother to daughter  ♂ cock-f. Sex-limited Trait Feathering pattern in chicken Genotype Males Females ♂ hen-f. HH hen feathering hen feathering Hh hen feathering hen feathering hh cock feathering hen feathering Cock feathering is an autosomal recessive trait ♀ hen-f. that is limited to males. Sex-influenced Trait The presence of a beard on some goats Genotype Males Females B+ B+ beardless beardless B + Bb bearded beardless Bb Bb bearded bearded Autosomal locus Bb is dominant in males and recessive in females. III. Sex-limitied & Sex-influenced Inheritance Autosomal genes whose expression depends on hormones - Sex-limited: expressed only in 1 gender - Sex-influenced: expressed by both genders, but in 1 more than the other X-linked Traits What indicates that a trait is determined by a gene that is not autosomal, but rather carried on chromosome X?  Perform reciprocal crosses i.e. reverse the traits between male & female  If the gene is autosomal, reciprocal crosses must yield the same results ♀ red-eyed X ♂ white-eyed ♀ white-eyed X ♂ red-eyed IV. X-linked Traits Reciprocal crosses do not yield the same results. Crisscross pattern of inheritance: recessive X-linked traits pass from homozygous/carrier mothers to all sons. - X-linked recessive disorders: daltonism (color-blindness) – hemophilia - X-linked dominant disorders: vitamin D-resistant rickets V. Hemizygosity Lygaus male (XY), Abraxas female (ZW), or Protenor male (XO) is hemizygous i.e. possesses only 1 copy of a gene in a rather diploid cell.  X-linked genes in males & females For many genes: male female Pairs of alleles gametes phenotypes genotypes homozygotes Heterozygotes n 2n 2n 3n 2n 3n – 2n Pairs of alleles gametes phenotypes genotypes homozygotes Heterozygotes n 2n 2n 2n - - For 1 gene polyallelism: male female Nb of alleles gametes genotypes homozygotes heterozygotes N N N(N+1)/2 N N(N-1)/2 Nb of alleles gametes genotypes homozygotes heterozygotes N N N - - (FS.II Final 2018) (FS.II Final 2018) (FS.II Final 2018) (FS.II Final 2018) (FS.II Final 2018) (FS.II Final 2018) Answer key 17) a 18) c 19) b 30) b 32) a 38) c 39) c 40) d