6/2/2015. (Sperm could also be XY)

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1 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) Example: snapdragon color Both red (RR) and white (rr) are dominant Heterozygous (Rr) = pink Use a Punnett square to predict the ratio of red:pink:white offspring if 2 pink snapdragons are crossed Genotype? 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 Genotype? Paternal alleles C S Maternal alleles S S 1

2 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 and codominance are NOT the same thing!! Incomplete dominance: phenotype is a blend of the two traits Dominant Dominant Codominance: both traits are seen at the same time Codominance Human example: A, B, O blood types Both type A and type B are dominant (I A and I B ) Make different glycoproteins on the membrane of red blood cells Type O is recessive Makes no such glycoprotein due to a frameshift mutation that produces a STOP codon If I A and I B are both present, both will be expressed Before the days of DNA testing, blood type was used to settle paternity suits Doesn t always work though Charlie Chaplin was involved in such a case in 1942 with actress Joan Barry 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 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

3 Charlie Chaplin s blood type: AB Only possible genotype: I A I B Joan Berry s blood type: O Only possible genotype: ii Child s blood type: O Only possible genotype: ii 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? 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 Trait 1: seed shape Dominant = spherical (S) Recessive = dented (s) Trait 2: seed color Dominant = yellow (Y) Recessive = green (y) Both plants to be crossed: SsYy Phenotype? What are the possible gamete combinations? (Each will get one allele of each gene) 3

4 Recombination Sometimes it appears that 2 traits on the same chromosome sort independently Recall the process of crossing over during meiosis DNA is replicated 4 copies of each chromatid form a tetrad Portions of homologous chromosomes are swapped Recombination 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 Chromosomal basis of gender discovered in 1905 by Nettie Stevens Homogametic: sex chromosomes are the same Heterogametic: sex chromosomes are not the same Homogametic sex is usually (but not always) default Humans (and most animals) Males = XY Females = XX Birds, some insects Females = ZW Males = ZZ 4

5 Chromosomal basis for gender discovered by studying 2 disorders: Turner s Syndrome (XO) Kleinfelter s Syndrome (XXY) Both caused by non-disjunction of sex chromosomes (Sperm could also be XY) Turner s Syndrome (XO) Only non-fatal complete monosomy Incidence: 1:2000 Short stature Non-functioning ovaries Webbed neck Swelling of hands and feet Skeletal abnormalities High blood pressure Heart defects Kidney problems Kleinfelter s Syndrome (XXY) Not a true trisomy, but 47 chomosomes present Incidence 1:500 1:1000 Highly varied Sometimes obvious at puberty Sometimes only discovered when seeking help for infertility Less testosterone = (sometimes) less muscle, poor beard growth, tall stature, breast development Mosaicism Chromosome non-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 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 Sex-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 Sex-Linked Inheritance Many genes on X chromosome Males are essentially monosomatic for these genes Whatever allele is present will be expressed Examples: hemophilia, muscular dystrophy, color blindness, many lysosomal storage diseases, Fragile X syndrome Fragile X Syndrome X-linked dominant disorder Variably expressed Results from >200 CGG repeats that lead to DNA methylation and silencing of FMR1 Causes physical and intellectual changes Nearly all children with Fragile X meet criteria for autism diagnosis Colorblindness 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 Male with normal color vision: X C Y - Female carrier with normal color vision: X C x C What is the probability that their child will be colorblind? X-Linked Recessive Pedigree Females are affected by X-linked disorders Far less common Consider the following pedigree for a family with colorblindness Assign genotypes to each individual How many carriers are there? 6