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6/2/2015 Incomplete Dominance Chapter 6 Genetics and Inheritance • Sometimes there is not one clear dominant allele • In a heterozygous individual, both alleles are expressed • Phenotype is a blend of both traits Lecture 2: Genetics and Patterns of Inheritance (continued) Incomplete Dominance • Both red (RR) and white (rr (rr)) are dominant • Heterozygous (Rr (Rr)) = pink • Use a Punnett square to predict the ratio of red:pink:white offspring if 2 pink snapdragons are crossed • Example in humans: hair • Both curly (CC) and straight (SS) are dominant • Heterozygous (CS) = wavy Use a Punnett square to predict the probability of a child with wavy hair from a father with wavy hair and a mother with straight hair Incomplete Dominance • Genotype? Maternal alleles S S S Incomplete Dominance • Genotype? C • Example: snapdragon color Paternal alleles • Incomplete Dominance 1 6/2/2015 Codominance • Incomplete vs. Codominance Commonly seen when more than 2 alleles exist for the same gene • Both dominant alleles are expressed at once • Not a blend of the 2 traits – both distinct traits can be seen at the same time • Incomplete dominance Dominant Dominant and codominance are NOT the same thing!! • Incomplete dominance: phenotype is a blend of the two traits • Codominance:: both Codominance traits are seen at the same time Codominance • • Human example: A, B, O blood types Both type A and type B are dominant • • • (IA and IB) Make different glycoproteins on the membrane of red blood cells Before the days of DNA testing, blood type was used to settle paternity suits • • Type O is recessive • • Chaplin Paternity Case Doesn’t always work though Charlie Chaplin was involved in such a case in 1942 with actress Joan Barry Makes no such glycoprotein due to a frameshift mutation that produces a STOP codon If IA and IB are both present, both will be expressed Chaplin Paternity Case • Charlie Chaplin’s blood type: AB • Joan Barry’s blood type: O • Child’s blood type: O • Use a Punnett square to determine whether Charlie Chaplin could have been the child’s father Chaplin Paternity Case • Charlie Chaplin’s blood type: AB • • • • Only possible genotype: Joan Berry’s blood type: O Only possible genotype: Child’s blood type: O • Only possible genotype: 2 6/2/2015 Chaplin Paternity Case • Charlie Chaplin’s blood type: AB • • • All of our examples of inheritance patterns have focused on single genes • Humans have 25,000 genes! • Genes on the same chromosome are inherited together • Genes on different chromosomes are inherited separately • If we consider just 2 chromosomes, how many different gametes could be produced by meiosis? IAIB Joan Berry’s blood type: O • • Only possible genotype: Independent Assortment Only possible genotype: ii Child’s blood type: O • Only possible genotype: ii Independent Assortment Independent Assortment • • 4 possible gametes produced Consider 2 pea plants • Heterozygous for 2 traits on 2 different chromosomes • Can create a dihybrid Punnett square to examine all offspring possibilities Independent Assortment • • Trait 1: seed shape • Dominant = spherical (S) • Recessive = dented (s) Trait 2: seed color • Dominant = yellow (Y) • Recessive = green (y) Independent Assortment • Both plants to be crossed: SsYy • • Phenotype? What are the possible gamete combinations? • (Each will get one allele of each gene) 3 6/2/2015 Recombination Recombination • Sometimes it appears that 2 traits on the same chromosome sort independently • DNA is replicated • Recall the process of crossing over during meiosis… • 4 copies of each chromatid form a tetrad • Portions of homologous chromosomes are swapped Recombination • This process is called recombination • Remember: 25,000 genes in human genome • Between recombination and independent assortment there are essentially infinite genetic combinations for the gametes of any individual Genetics of Gender • Chromosomal basis of gender discovered in 1905 by Nettie Stevens Genetics of Gender Genetics of Gender • Homogametic: sex chromosomes are the same • Heterogametic: sex chromosomes are not the • Males = XY same • Females = XX • • Humans (and most animals) Homogametic sex is usually (but not always) default • Birds, some insects • Females = ZW • Males = ZZ 4 6/2/2015 Genetics of Gender • • Chromosomal basis for gender discovered by studying 2 disorders: • Turner’s Syndrome (XO) • Kleinfelter’s Syndrome (XXY (XXY)) Both caused by nonnon-disjunction of sex chromosomes (Sperm could also be XY) Turner’s Syndrome (XO) Kleinfelter’s Syndrome (XXY) • Only nonnon-fatal complete monosomy • Not a true trisomy, trisomy, but 47 chomosomes present • Incidence: 1:2000 • Incidence 1:500 – 1:1000 • Short stature • Highly varied • Non--functioning ovaries Non • Sometimes obvious at puberty • Webbed neck • Sometimes only discovered • Swelling of hands and feet • Skeletal abnormalities • High blood pressure (sometimes) less muscle, • Heart defects poor beard growth, • Kidney problems tall stature, breast development when seeking help for infertility • Mosaicism • Chromosome nonnon-disjunction takes place in mitosis during embryonic development • Some cells will have normal chromosome counts • Some cells will have 45 or 47 chromosomes • Extreme case: hermaphroditism Less testosterone = Mosaicism • Also occurs normally with X chromosomes • Called X silencing • One X chromosome is silenced in every cell of the female body • Which X is silenced depends on the cell • Usually not noticeable • Example: calico cat 5 6/2/2015 SexSex -Linked Inheritance • • When one gender is affected by a genetic disorder more than the other In humans = most often males • X chromosome = large • Y chromosome = small SexSex -Linked Inheritance Fragile X Syndrome • Many genes on X chromosome • X-linked dominant disorder • Males are essentially monosomatic for these genes • Variably expressed • Results from >200 CGG • Whatever allele is present will be expressed repeats that lead to DNA • Examples: hemophilia, muscular dystrophy, color blindness, many lysosomal storage diseases, Fragile X syndrome methylation and silencing of FMR1 • Causes physical and intellectual changes • Nearly all children with Fragile X meet criteria for autism diagnosis Colorblindness X-Linked Recessive Pedigree • Genes for color vision are on X chromosome • No corresponding gene on Y chromosome • Normal color vision is dominant, so males are most often affected • Consider the following pedigree for a family with colorblindness • Consider… • Assign genotypes to each individual • How many carriers are there? XCY- • Male with normal color vision: • Female carrier with normal color vision: XCxC • • Females are affected by XX-linked disorders • Far less common What is the probability that their child will be colorblind? 6