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Ch. 7: Extending Mendelian Genetics 7.1 KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits. Ch. 7: Extending Mendelian Genetics Two copies of each autosomal gene affect phenotype. • Mendel studied autosomal gene traits, like hair texture. Ch. 7: Extending Mendelian Genetics • Mendel’s rules of inheritance apply to autosomal genetic disorders. – Recessive disorder requires a homozygous recessive genotype to affect a person. A heterozygote for a recessive disorder is a carrier. – Disorders caused by dominant alleles are uncommon. (dominant) Ch. 7: Extending Mendelian Genetics • Codominant alleles • Two equally dominant alleles are expressed at the same time. • Heterozygous phenotype will have both phenotypes visible Ch. 7: Extending Mendelian Genetics Shorthorn Cattle • Co- dominance • Homozygous red (RR) • Homozygous white (WW) The offspring of will have both red and white hairs (RW) The offspring are heterozygous and called “roan” Ch. 7: Extending Mendelian Genetics Sickle- Cell Anemia • Co- dominance • Caused by an abnormal Hemoglobin, the protein that red blood cells use to carry oxygen Normal hemoglobin is (RR) Sickle Cell shaped blood cells (SS) People who are carriers (heterozygous) for the disease there is a mixture of both normal and sickle cell (RS) Ch. 7: Extending Mendelian Genetics • In incomplete dominance • Neither allele is completely dominant over the other allele. • A heterozygous phenotype – A mixture or blending of the two Ch. 7: Extending Mendelian Genetics Four-o’ clock flowers • Incomplete dominance • Neither Red (R) • or White (W) is dominant When a homozygous red flower (RR) Mix with a homozygous white flower (WW), the alleles blend in the hybrid (RW) to produce pink flowers Ch. 7: Extending Mendelian Genetics Andalusian Chickens • Incomplete dominance • Neither Black (B) or White (W) are dominant The offspring of a black feathered chicken (BB) and a white feathered chicken (WW) are blue (BW) – BLUE HEN!! Ch. 7: Extending Mendelian Genetics • Mendel’s rule only apply for autosomal genes(chromosome pairs 1-22). Genes on sex chromosomes are called sexlinked genes. – Y chromosome genes in mammals are responsible for male characteristics. Males have an XY genotype. – X chromosome genes in mammals affect many traits. Females have an XX genotype. Ch. 7: Extending Mendelian Genetics – Because males only have one copy of the X chromosome all of a male’s sexlinked genes are expressed. – In females, expression of sexlinked genes is similar to autosomal genes because they have two copies of the X chromosome. Ch. 7: Extending Mendelian Genetics – Common examples of sex-linked disorders: • Duchenne’s muscular dystrophy • Hemophilia • Color Blindness Ch. 7: Extending Mendelian Genetics Ch. 7: Extending Mendelian Genetics Normal Color Vision Red-Green Color Blind Left Right Left Right Top 25 29 Top 25 Spots Middle 45 56 Middle Spots 56 Bottom 6 8 Bottom Spots Spots The test to the left is simpler. The individual with normal color vision will see a 5 revealed in the dot pattern. An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots. Ch. 7: Extending Mendelian Genetics – X chromosome inactivation randomly “turns off” one X chromosome. Ch. 7: Extending Mendelian Genetics • Codominant alleles will both be completely expressed. – Codominant alleles are neither dominant nor recessive. – The ABO blood types result from codominant alleles. • Many genes have more than two alleles. Ch. 7: Extending Mendelian Genetics • There are 3 alleles that determine blood type: IA, IB, and i – IA and IB are dominant to i. – IA and IB are codominant. Ch. 7: Extending Mendelian Genetics Codominance Ch. 7: Extending Mendelian Genetics – There are 4 blood types: A: • Genotype: IAIA or IAi • Antigen: A • Antibody: Anti-B • Can receive from: A, O • Can donate to: A, AB Ch. 7: Extending Mendelian Genetics B: • Genotype: IBIB or IBi • Antigen: B • Antibody: Anti-A • Can receive from: B, O • Can donate to: B, AB Ch. 7: Extending Mendelian Genetics AB : • Genotype: IAIB • Antigen: A & B • Can receive from: A, B, AB, & O (universal recipient) • Antibody: none • Can donate to: AB Ch. 7: Extending Mendelian Genetics O: • Genotype: ii • Antigen: none • Antibody: Anti-A and Anti-B • Can receive from: O • Can donate to: A, B, AB, & O (universal donor) Ch. 7: Extending Mendelian Genetics • Rh Factor: Another surface marker which may or may not be present on the RBC. Ch. 7: Extending Mendelian Genetics Rh+ : • Genotype: Rh+Rh+ or Rh+Rh • - Can receive from: Rh+ & Rh• Antigen: Rh • Can donate to: Rh+ • Antibody: none Ch. 7: Extending Mendelian Genetics Rh- : • Genotype: Rh-Rh• Antigen: none • Antibody: Rh+ • Can receive from: Rh- • Can donate to: Rh- & Rh+ Ch. 7: Extending Mendelian Genetics • Many genes have more than two alleles or multiple alleles used to express a trait. Ch. 7: Extending Mendelian Genetics • Polygenic traits are produced by two or more genes. Order of dominance: brown > green > blue. Ch. 7: Extending Mendelian Genetics Ch. 7: Extending Mendelian Genetics • An epistatic gene can interfere with other genes. Ch. 7: Extending Mendelian Genetics Epistatic: recessive trait blocks another allele Black is dominant to chocolate B or b Yellow is recessive epistatic (E or e) Phenotype Possible Genotypes BBEE BbEE BBEe BbEe bbEE bbEe BBee Bbee bbee Ch. 7: Extending Mendelian Genetics Phenotype can be a combination of genotype and environment. Malnourished Fed and cared for. Identical twins separated at birth and treated differently. Ch. 7: Extending Mendelian Genetics 7.4 KEY CONCEPT A combination of methods is used to study human genetics. Ch. 7: Extending Mendelian Genetics A pedigree is a chart for tracing genes in a family. • Phenotypes are used to infer genotypes on a pedigree. • Autosomal genes show different patterns on a pedigree than sex-linked genes. Ch. 7: Extending Mendelian Genetics • Squares represent males and circles females. • A coloured in shape means that person has the trait in question. • A half coloured in shape means that they are carrying an allele for a recessive trait. Ch. 7: Extending Mendelian Genetics • If the phenotype is more common in males, the gene is likely sex-linked. Ch. 7: Extending Mendelian Genetics 7.3 KEY CONCEPT Genes can be mapped to specific locations on chromosomes. Ch. 7: Extending Mendelian Genetics • Because of their location on a chromosome, some genes travel together and are considered to be linked. Wild type Mutant Ch. 7: Extending Mendelian Genetics Linkage maps estimate distances between genes. • The closer together two genes are, the more likely they will be inherited together. • Linkage maps show the relative locations of genes. Ch. 7: Extending Mendelian Genetics