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Bio11 – Announcements TODAY – Genetics Test 2 AVG = 73 pts Ch. 20 Patterns of Inheritance Test scores posted Next week’s lab: Case study presentations Extra credit due Thur Mar 3 How traits are passed from generation to generation 15 pts available, see my website Human Chromosomes DNA packing in chromosomes 46 chromosomes in pairs 22 pairs of matching chromosomes Plus 1 pair of sex chromosomes XX or XY 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. This is a human karyotype Male or female? Chromosomes and genes Each chromosome contains one very long DNA molecule Typically bears thousands of genes Genes carry our traits What is heredity? Genes are segments of DNA 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 Genotype and phenotype Genes carry our traits Genes found at specific locations on a chromosome Alleles are different versions of a gene 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 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 F Phenotype – what an individual looks like 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 Widow’s peak For each trait, we inherit two alleles, one from each parent. Straight hairline F Gene “F” codes for freckles Examples of traits controlled by a single gene in humans Genotype and phenotype Genotype – an individual’s genes 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 Inheriting a trait More about alleles Alleles can be dominant or recessive. The dominant allele determines the organism’s appearance Phenotypes: Dad Mom Use upper case: P The recessive allele has no noticeable effect If a man with short fingers marries a woman with long fingers, what genotypes and phenotypes will their children have? Genotypes? PP or Pp pp Use lower case: p Dominant trait SS or Ss Forming the gametes Forming the gametes Gametes carry only one allele for each inherited characteristic. Dad Gametes carry only one allele for each inherited characteristic. Mom S S S meiosis Genotype = SS s s S S s meiosis Gametes only carry one copy of each chromosome Genotype = ss Fertilization Recessive trait ss Forming the gametes When sperm and egg unite at fertilization, each contributes its allele. What if dad is Ss? Dad S s Fertilized egg S S s meiosis Genotype = Ss Genotype = Ss Phenotype? S s s S Then after fertilization … Dad Mom s S s 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 Gametes: S or s All s no freckles no freckles ff ff meiosis Fertilized egg: genotype? phenotype? Ss or ss gametes Offspring f f Offspring ffff no freckles Crosses (more complicated) If both parents are heterozygous for freckles, what will be the genotype of their offspring? Punnett square analysis Used to predict the genotypes of the offspring #1 Decide what gametes would be produced by each parent. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents no freckles Freckles ffFf Ff ff F Genotype? Gametes? no freckles Freckles Mom Ff F f Dad Ff F f f F meiosis gametes f Gametes? f Gametes? Offspring f ff no freckles Punnett square analysis What are the genotypes of the offspring of two heterozygous freckled parents (Ff)? F f F FF Ff f Ff Ratio: 1FF, 2 Ff, 1 ff ff What are the phenotypes? Human Disorders Controlled by a Single Gene Recessive disorders Recessive disorders Most human genetic disorders are recessive. Albinism Cystic fibrosis lack of melanin pigment in the eyes, skin and hair affects mammals (including humans), fish, birds, reptiles and amphibians 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. Albino alligator 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 Incomplete Dominance: Hypercholesterolemia appearance is between the phenotypes of the two parents. Alleles are not fully dominant or fully recessive Multiple Alleles: ABO blood groups Three alleles for the same gene Both alleles are expressed in the heterozygote. 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 Inheritance of colorblindness Sex-linked disorders IA = A antigen on RBC IB = B antigen on RBC i = neither A or B antigen Sex-linked genes The I A and IB alleles exhibit codominance. Characterized by dangerously high levels of cholesterol in the blood due to recessive alleles seen mostly in males Example Red-green color blindness characterized by a malfunction of lightsensitive cells in the eyes. 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 Sex-linked disorders: Duchenne muscular dystrophy Set up a Punnett square XB Mom = XB , X b Xb Dad = XB , Y XB Possible genotypes and phenotypes of Sons? Daughters? Y Sex-linked disorders: Hemophilia X-linked recessive disorder Symptoms: Wasting away of the muscles Inheritance of colorblindness Eventually confined to wheelchair Death by age 20 Hemophilia: an X-linked, recessive trait Two genes that encode blood-clotting proteins are on the X chromosome Due to absence of protein involved in release of Ca from ER in muscles 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 Any male (XY) who inherits the mutant gene will develop hemophilia Rare: occurs in 1/10,000 Caucasian males 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