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1 Biology 102 Lecture 10: Chromosomes and Sex Inheritance 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 This is what you will do in Lab 5 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: Phenotype? What are the possible gamete combinations? (Each will get one allele of each gene) 1

2 Gametes for Parent 2 SY Sy sy sy SY SsYY SSYy SSYY Gametes for Parent 1 Sy Ssyy SSyy SSYy sy ssyy ssyy Yellow) SsYY sy ssyy ssyy Yellow) Ssyy Phenotypic ratio? Spherical yellow = 9/16 SSYY, SsYY, SSYy, Spherical green = 3/16 SSyy, Ssyy Dented yellow = 3/16 ssyy, ssyy Dented green = 1/16 ssyy 9:3:3:1 inheritance pattern typical for straight- forward complete dominance situations Recombination 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 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 2

3 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 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: 1:2-3,000 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 chromosomes 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 3

4 Trisomy X Syndrome (XXX) Another sex chromosome trisomy Occurrence: 1 in 1,000 women No noticeable defects Tendency to be tall, learning disabled Fertile, offspring have no higher chance of trisomy Additional mechanism must prevent inheritance of additional X Jacob Syndrome (XYY) Another sex chromosome trisomy Occurrence: 1 in 1,000 men High levels of testosterone Tall Severe acne Normal intelligence Delayed emotional maturity (insert joke here) 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 Males only have one X chromosome, yet function normally Remember: most monosomies are fatal Only one copy of the genes on this chromosome must be required But females have two X chromosomes How does the body deal with competing expression? Mosaicism X-Inactivation All females are technically mosaics! One X chromosome is silenced in every cell of the female body Called X silencing or X inactivation Forms collapsed structure called a Barr body Which X is silenced depends on the cell Usually not noticeable Example: calico cat 4

5 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 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 (Y - ) Normal color vision is dominant, so males are most often affected Proper notation: Normal allele: X C Colorblind allele: X c Really difficult to tell the difference on PowerPoint We will use X C for normal, X cb for colorblind Colorblindness Colorblindness Consider Male with normal color vision: X C Y - Female carrier with normal color vision: X C X cb What is the probability that their child will be colorblind? Genotypes? Paternal alleles X C Y - Maternal alleles X C X cb 5

6 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? X-Linked Recessive Pedigree 1. Assign genotypes to affected individuals Circles are females Squares are males 2. Assign genotype to unaffected males 3. Assign one normal allele to unaffected females 4. Work backwards to determine missing alleles (if possible) 5. How many carriers are there in the pedigree? Can you tell? 6