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Bio11 – Announcements TODAY – Genetics Test 2 AVG = 73 pts Test scores posted Next week’s lab: Case study presentations Extra credit due Thur Mar 3 15 pts available, see my website Ch. 20 Patterns of Inheritance How traits are passed from generation to generation Human Chromosomes 46 chromosomes in pairs 22 pairs of matching chromosomes Plus 1 pair of sex chromosomes XX or XY This is a human karyotype Male or female? DNA packing in chromosomes Chromosomes contain DNA and protein The long strands of DNA are condensed The DNA is packed into an elaborate, multilevel system of coiling and folding. Chromosomes and genes Each chromosome contains one very long DNA molecule Typically bears thousands of genes Genes carry our traits Genes are segments of DNA What is heredity? Heredity – passing traits from parent to offspring The genes for certain traits are passed down in families from parents to children. For example, parents with black hair will have kids with black hair Tall parents will have tall kids Genetics 101: Where do your genes come from? (4:14) http://www.youtube.com/watch?v=lJzZ7p-47P8&NR=1 Genes carry our traits Genes found at specific locations on a chromosome Alleles are different versions of a gene Genotype and phenotype Genotype – an individual’s genes Gene “F” codes for freckles Phenotype – what an individual looks like F F Genotype and phenotype What is her genotype? The gene for freckles has 2 alleles The dominant allele F codes for freckles The recessive allele f does not She has two copies of each chromosome Possible genotypes: FF or Ff F F F f Examples of traits controlled by a single gene in humans These 3 traits are determined by simple dominant-recessive inheritance Possible genotypes for someone with freckles? FF and Ff Without freckles? ff More about alleles They are alternative versions of the same gene The gene for hairline has 2 alleles P for Widow’s peak p for straight hairline Widow’s peak Straight hairline More about alleles For each trait, we inherit two alleles, one from each parent. The alleles can be the same or different If they are different, the organism is heterozygous – Pp If they are the same, the organism is homozygous for that gene – PP, pp, YY Widow’s peak Straight hairline More about alleles Alleles can be dominant or recessive. The dominant allele determines the organism’s appearance Phenotypes: Use upper case: P The recessive allele has no noticeable effect Use lower case: p Genotypes? PP or Pp pp Inheriting a trait If a man with short fingers marries a woman with long fingers, what genotypes and phenotypes will their children have? Dad Dominant trait SS or Ss Mom Recessive trait ss Forming the gametes Gametes carry only one allele for each inherited characteristic. Dad S S S S meiosis Genotype = SS S Gametes only carry one copy of each chromosome Forming the gametes Gametes carry only one allele for each inherited characteristic. Mom s s s meiosis Genotype = ss Fertilization When sperm and egg unite at fertilization, each contributes its allele. S s Fertilized egg S s Genotype = Ss Phenotype? Forming the gametes What if dad is Ss? Dad S S s s meiosis Genotype = Ss S Then after fertilization … Dad S Gametes: Fertilized egg: genotype? phenotype? Mom s S or s Ss or ss s s All s Crosses (one-trait inheritance) If neither parent has freckles, what will be the genotype of their offspring? Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents no freckles no freckles ff ff meiosis gametes Offspring f Offspring f ffff no freckles Crosses (more complicated) If both parents are heterozygous for freckles, what will be the genotype of their offspring? Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents no freckles Freckles Ff ff no freckles Freckles ffFf meiosis gametes f Gametes? f Gametes? Offspring ff no freckles Punnett square analysis Used to predict the genotypes of the offspring #1 Decide what gametes would be produced by each parent. F Genotype? Gametes? Mom Ff F f F Dad Ff F f f f Punnett square analysis What are the genotypes of the offspring of two heterozygous freckled parents (Ff)? F F f FF Ff Ratio: 1FF, 2 Ff, 1 ff f Ff ff What are the phenotypes? Human Disorders Controlled by a Single Gene Recessive disorders Most human genetic disorders are recessive. Albinism Albino alligator lack of melanin pigment in the eyes, skin and hair affects mammals (including humans), fish, birds, reptiles and amphibians Recessive disorders Cystic fibrosis Most common lethal genetic disease in US Symptoms: excessive secretion of a very thick mucus which interferes with breathing Symptoms usually appear shortly after birth. Dominant Disorders Some human genetic disorders are dominant. Achondroplasia is a common form of dwarfism. Cause: abnormal bone and cartilage formation Huntington’s disease: a dominant genetic disorder Caused by a dominant allele–every individual who carries the allele gets the disorder Fatal: causes progressive deterioration of the brain Late age of onset: most people do not know they are affected until they are more than 30 years old Many neurons in normal brain. Loss of neurons in Huntington brain. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Some traits are controlled by multiple genes At least 180 genes control how tall a person will grow Beyond simple inheritance Incomplete Dominance The heterozygotes have an intermediate phenotype appearance is between the phenotypes of the two parents. Alleles are not fully dominant or fully recessive Incomplete Dominance: Hypercholesterolemia Characterized by dangerously high levels of cholesterol in the blood Multiple Alleles: ABO blood groups Three alleles for the same gene IA = A antigen on RBC IB = B antigen on RBC i = neither A or B antigen The IA and IB alleles exhibit codominance. Both alleles are expressed in the heterozygote. Sex-linked genes Genes located on a sex chromosome, usually the X chromosome. Their pattern of inheritance reflects the fact that females have two X chromosomes, but males have only one. Sex-Linked Disorders in Humans Sex-linked disorders due to recessive alleles seen mostly in males Example Red-green color blindness characterized by a malfunction of lightsensitive cells in the eyes. Inheritance of colorblindness An X-linked, recessive disorder XB = normal vision Xb = color blindness Write the following genotypes Normal man Color-blind man Normal woman Woman carrier Inheritance of colorblindness A man with normal vision and a woman carrier have children. What is the chance that the couple will have a colorblind daughter? A color-blind son? Man’s genotype Man’s gametes Woman’s genotype Woman’s gametes Inheritance of colorblindness Set up a Punnett square XB Xb Mom = XB , Xb Dad = XB , Y XB Y Possible genotypes and phenotypes of Sons? Daughters? Sex-linked disorders: Duchenne muscular dystrophy X-linked recessive disorder Symptoms: Wasting away of the muscles Eventually confined to wheelchair Death by age 20 Due to absence of protein involved in release of Ca from ER in muscles Sex-linked disorders: Hemophilia Hemophilia: an X-linked, recessive trait Two genes that encode blood-clotting proteins are on the X chromosome Any male (XY) who inherits the mutant gene will develop hemophilia Rare: occurs in 1/10,000 Caucasian males Family pedigrees Used to find out if a particular human trait is inherited geneticists can’t control the mating of their subjects, so they analyze the results of matings that have already occurred Assemble info into a family tree represent males, females Colored symbols = person has trait being studied Practice problems Prepare for the quiz on Thur Questions at end of Chapter 20: #1, 3, 7, 11, 13, 15, 16, 19, 20, 26, 27 and 30