2Human Genes and Chromosomes 31 thousand genes located over 46 chromosomesAutosomes22 pairs (44 total)Sex Chromosomes1 pair (2 total)XX = femaleXY = maleSome genes are located on the sex chromosomes….they are called “sex-linked”
3Sex Determination: -will a child be male or female? (XX)male(XY)eggsspermYXXxXXXYXY
4Men show sex-linked traits more often… X-linked recessive traits that are not related to feminine body characteristics are primarily expressed in the observable characteristics, or phenotype , of men. This is due to the fact that men only have one X chromosome. Subsequently, genes on that chromosome not coding for gender are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome in most cases. In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other. This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes.Why?
5Sex-linked genes…aka: X-linked genes HemophiliaColorblindnessDuchenne Muscular Dystrophy**Must take into consideration the X and the Y chromosomes when doing these problems**We refer to X-linked genes with superscriptsex: XHXh=female carrier of hemophiliaXhXh=female hemophiliacXh Y =male hemophiliacWhy are most of these genes located on the Xchromosome?
7HemophiliaThe X chromosome carries genes that help control blood clotting.In hemophilia, a protein necessary for normal blood clotting is missing.Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
8Practice Problem:What are the chances for a mother that is a carrier for hemophilia and a hemophiliac father to have hemophiliac children?
9Color blindness:Three human genes associated with color vision are located on the X chromosome.In males, a defective version of any one of these genes produces colorblindness.Father(normal vision)Normal visionColorblindMaleFemaleDaughter(normal vision)Son(normal vision)Mother (carrier)Daughter(carrier)Son(colorblind)Go to Section:
10Duchenne muscular dystrophy Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle.It is caused by a defective version of the gene that codes for a muscle protein.
11Pedigrees and sex-linked traits: Terms to know:Normal= unaffected with diseaseunshaded in pedigreesAffected= has diseasefully shaded in pedigreesSquare= maleCircle= femaleHemophilia(heem-o-FILL-ee-ah) is a very rare disease in which your blood clots abnormally. It is an inherited disease, which means that it has to be passed on from another ancestor or relative. It has to do with mostly blood clotting. If a Hemophilia patient bleeds, especially from an injury, they can bleed fairly long and there is a huge loss of blood from the body. Now, platelets need to be injected into the body. As I said before, this is a fairly rare disease, especially for males. "About 1 in 5,000 boys have Hemophilia." (1)Therefore, females aren't as likely to get the gene. People who have Hemophilia have to be extremely careful and cautious about not falling down or even just getting a paper cut.
12Practice Problem: This pedigree shows the inheritance of hemophilia: Find the phenotypes for #1, #2, and #4If #5 marries a woman who is heterozygous for the hemophiliac gene, XHXh, what are their chances for having children with hemophilia?
13Genetic Disorders: Autosomal Caused by Recessive AllelesRecessive disorders only appear if two recessive alleles are present.AlbinismCystic fibrosisGalactosemia
14Cystic FibrosisSufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways.The most common allele that causes cystic fibrosis is missing 3 DNA bases.As a result, the amino acid phenylalanine is missing from the CFTR protein.Normal CFTR is a chloride ion channel in cell membranes.Abnormal CFTR cannot be transported to the cell membrane.The cells in the person’s airways are unable to transport chloride ions.As a result, the airways become clogged with a thick mucus.
15Genetic Disorders: Autosomal Caused by Dominant Alleles :Dominant disorders are caused by dominant alleles, so only one allele is needed for the disease to show.AchondroplasiaHuntington’s DiseaseHypercholesterolemia
16Genetic Disorders: Chromosomal Caused by NondisjunctionDown Syndrome—Trisomy Chromosome 21Down syndrome produces mild to severe mental retardation.It is characterized by:increased susceptibility to many diseaseshigher frequency of some birth defectsKaryotype: picture of human chromosomes
17Changes in Chromosome Number usually the result of Nondisjunction chromosome alignments at metaphase In - 1nondisjunction at anaphase Ialignments at metaphase IIanaphase II
18Nondisjunction in Sex Chromosomes Turner’s Syndrome—Monosomy Sex Chromosome (X0)A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).Women with Turner’s syndrome are sterile.Klinefelter’s Syndrome—Trisomy Sex Chromosome (XXY)The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.
19Pedigree Analysis Technique to study patterns of human inheritance Visual representation of family historySeveral possible modes of inheritance are able to be identified:Autosomal dominant traitsAutosomal recessive traitsSex-linked traits
20What information do we find in a pedigree? Horizontal lines between a male and a female = marriageVertical lines coming down from a marriage = offspringFemale affectedMale affectedFemale with “normal” traitMale with “normal” trait
21Autosomal Dominant Appears in both sexes with equal frequency Can be passed on to the next generation by both males and femalesDoes not skip a generationEx: Huntington’s Disease
22Autosomal Recessive Appears at equal frequency in both sexes. Only appears when the affected individual has received one allele from each parentSkips generationsEx: Cystic Fibrosis
23Sex-linkedmore frequent in malestrait tends to skip generations
24MCAS practice question People who are tune deaf are unable to follow a rhythm. Scientists have evidence that tune deafness can be genetic. The pedigree below traces the inheritance of tune deafness in a family. Individuals in the pedigree are numbered. Scientists have analyzed the inheritance patterns for tune deafness and have concluded that tune deafness is caused by an autosomal dominant allele, TProvide evidence from the pedigree that shows that the tune deafness allele is autosomal dominant, not autosomal recessive. Explain.Identify the genotypes of individuals 5 and 6, and then draw the Punnett square for the cross of these two individuals.
25CASE STUDY: ABO Blood Typing What inheritance patterns does human blood type demonstrate?
26ABO GeneDetermined by a single gene with 3 alleles, 2 of which are codominantType A Blood (type A marker)Makes antibody for BType B Blood (type B marker)Makes antibody for AType AB Blood (both A & B markers)Makes no antibodiesType O Blood (neither marker)Makes antibodies for A and B
28Risks of Blood Transfusions Donor RBCs may not have the same kind of recognition molecules (“markers”) on their surfaces as Recipient RBCsPotential Result:Agglutination, a defense response. CLUMPINGAntibodies act against foreign cells and cause them to clump togetherAntibodies are produced against antigens NOT present on RBCs
29When does agglutination occur? Type Aantibodies ignore A marker, attack B markersType Bantibodies ignore B marker, attack A markersType ABantibodies ignore both formsType Oantibodies attack both A and B markers
30Blood Transfusions and Agglutination Safe TransfusionsPhenotype(Blood Type)Antigen onRed Blood CellGenotypeToFromGo to Section:
31Blood Typing in Forensics Can be used to determine the blood type of a potential suspect in a crimeTest uses two solutions each containing antibodies to type A and type B antigens.Solution 1: Anti-A; when mixed with type A blood will cause it to form clumps.Solution 2: Anti-B; when mixed with type B blood will cause it to form clumps.If blood clumps under contact with both Anti-A and Anti-B, then it is type ABO blood does not clump
32Rh Gene Rh marker determined by a single gene with 2 alleles Rh+ allele=Dominant=marker presentRh- allele=recessive=marker not present