Beyond Mendel’s Laws of Inheritance 2006-2007

Incomplete dominance Heterozygote shows an intermediate, blended phenotype example: RR = red flowers rr = white flowers Rr = pink flowers make 50% less color RR WW RW RR RW WW

Incomplete dominance P F1 100% 1:2:1 F2 X true-breeding red flowers white flowers 100% 100% pink flowers F1 generation (hybrids) self-pollinate 25% white F2 generation 25% red 1:2:1 50% pink

Codominance There are two or more alleles that are dominant in a phenotype. Both alleles are expressed in heterozygous condition. Ex: Red – RR, white – WW, red and white - RW

Multiple Alleles 2 alleles affect the phenotype equally & separately not blended phenotype human ABO blood groups 3 alleles IA, IB, i IA & IB alleles are co-dominant glycoprotein antigens on RBC IAIB = both antigens are produced i allele recessive to both

Polygenic inheritance Some phenotypes determined by additive effects of 2 or more genes on a single characteristic phenotypes on a continuum human traits skin color height weight intelligence behaviors

Sex linked traits 1910 | 1933 Genes are on sex chromosomes as opposed to autosomal chromosomes first discovered by T.H. Morgan at Columbia U. Drosophila breeding good genetic subject prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male

Discovery of sex linkage true-breeding red-eye female true-breeding white-eye male X P Huh! Sex matters?! 100% red eye offspring F1 generation (hybrids) 100% red-eye female 50% red-eye male 50% white eye male F2 generation

Genetics of Sex X Y X XX XY X XX XY In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develop as a female: XX gene redundancy, like autosomal chromosomes an X & Y chromosome develop as a male: XY no redundancy X Y X XX XY X XX XY 50% female : 50% male

Morgan’s flies… x x XRXR XrY XRXr XRY Xr Y XR Y XR XR XRXr XRY XRXR 100% red females 50% red males; 50% white males 100% red eyes

Genes on sex chromosomes Y chromosome few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones X chromosome traits other than sex determination mutations: (all are recessive) hemophilia Duchenne muscular dystrophy color-blindness Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. The disease is characterized by a progressive weakening of the muscles and loss of coordination.

Hemophilia XHXh XHY Hh x HH XH XHXh XH Y Xh XHXH XHXH XHY XHY XH Xh XH sex-linked recessive Hemophilia XHXh XHY Hh x HH XH XHXh XH Y male / sperm Xh XHXH XHXH XHY XHY XH Xh female / eggs XH XHY XHXh XHXh XhY XhY Y carrier disease

X-inactivation Female mammals inherit 2 X chromosomes XH XHXh Xh one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random patchwork trait = “mosaic” patches of black XH XHXh Xh tricolor cats can only be female patches of orange

Environmental effects Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences Color of Hydrangea flowers is influenced by soil pH

Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees

(a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww ww 3rd generation (two sisters) WW ww Figure 14.15a Pedigree analysis or Ww Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?

(b) Is an attached earlobe a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) ff FF or Ff Figure 14.15b Pedigree analysis Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?