Genetics ppt 2
Beyond Mendel’s Laws of Inheritance AP Biology 2006-2007 Incomplete dominance Heterozygote shows an intermediate, blended phenotype example: RR = red flowers RR rr = white flowers WW Rr = pink flowers RW make 50% less color AP Biology RR RW WW Incomplete dominance P X true-breeding red flowers true-breeding white flowers 100% pink flowers F1 100% generation (hybrids) self-pollinate 25% red F2 generation AP Biology 50% pink 25% white 1:2:1 Codominance There are two or more alleles that are dominant in a phenotype. Both alleles are expressed in heterozygous condition. AP Biology 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 AP Biology 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 AP Biology 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 AP Biology Discovery of sex linkage P F1 true-breeding red-eye female X true-breeding white-eye male 100% red eye offspring Huh! Sex matters?! generation (hybrids) F2 generation AP Biology 100% red-eye female 50% red-eye male 50% white eye male Genetics of Sex 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 X Y X XX XY X XX XY develop as a male: XY no redundancy AP Biology 50% female : 50% male Morgan’s flies… x XR XR Xr XR XR AP Biology XR Xr XR Xr x XrY XR Xr Y XRY XRY 100% red eyes XR Xr XRY XR Y XR XR XRY XR Xr X rY 100% red females 50% red males; 50% white males 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 AP Biology sex-linked recessive Hemophilia H Xh x X HY HH XHh XH female / eggs male / sperm XH XH Y XH XH XH Y XH Xh Xh XH Xh AP Biology XH Xh XhY carrier disease XHY Y X-inactivation Female mammals inherit 2 X chromosomes 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 XH Xh tricolor cats can only be AP Biology female Xh 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 Color of Hydrangea flowers APinfluenced Biology is 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 AP Biology Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) Ww ww ww Ww ww ww Ww Ww Ww ww 3rd generation (two sisters) WW or Ww Widow’s peak ww No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? AP Biology Fig. 14-15c 1st generation (grandparents) Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff Ff ff ff Ff Ff Ff ff ff FF or Ff 3rd generation (two sisters) Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? AP Biology
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