Chapter 10 – Patterns of Inheritance
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Chapter 10 – Patterns of Inheritance Why do we look like our parents? What do we inherit from them? History of genetics Gregor Mendel Austrian monk who in 1860 developed theories on inheritance His organism of study was the pea plant His theories came from the careful fertilization of these plants & looking at the characteristics of the offspring (color, seed texture, etc) Then utilizing a little MATH, he came up with formulas & theories on how parents transmit characteristics to their offspring The neat thing! His theories came before our understanding of meiosis Chapter 10 – Patterns of Inheritance Mendel’s Laws The law of segregation Each individual has two factors for each trait The factors segregate (separate) during the formation of gametes Each gamete contains only one factor from each pair of factors Fertilization gives each new individual two factors for each trait The law of independent assortment Each pair of factors segregates independently of the other pairs. Remember MEIOSIS All possible combinations of factors can occur in the gametes Before we look at these laws there are some terms that must first be clarified! Chapter 10 – Patterns of Inheritance What is a gene? A sequence of DNA that encodes for a trait What is an allele? An alternative form of a gene Thus, if you have a hair color gene, you could have a brown allele & a blonde allele to determine brown vs. blonde hair How are these alleles represented in your chromosome pairs? As a pair, so if the hair color genes are located on chromosome #1, each of your chromosome #1s will have an allele for hair color So if “B” = brown allele & “b” = blonde allele, your possible combinations are: BB, Bb, bb = these are your genotypes = the genetic make-up for this trait Thus if you are BB, you only have brown alleles, then your hair color must be BROWN = phenotype = expressed or visible trait linked to your genotype Chapter 10 – Patterns of Inheritance But what would your phenotype be if you were Bb? Genotype terminology Homozygous = the two alleles in a pair are identical Heterozygous = the two alleles in a pair are different Thus a Bb person has a heterozygous genotype, but what would his phenotype be? Dominant vs. Recessive relationship between alleles If a Bb person has brown hair, then the B allele is dominant to the b allele & the dominant phenotype will manifest itself in these heterozygous individuals There are other allele relationships which we will discuss later in the chapter Let’s look at some genetic problems to illustrate these concepts Chapter 10 – Patterns of Inheritance What will be the composition of a parents alleles given the following composition? BB Bb Thus a parent gives up only one allele for a given trait to their offspring! bb Chapter 10 – Patterns of Inheritance How will the offspring inherit these traits & what will they look like? One trait or monohybrid crosses – Utilization of the Punnett Square Homozygous dominant X Homozygous recessive Chapter 10 – Patterns of Inheritance How will the offspring inherit these traits & what will they look like? Heterozygous X Heterozygous Chapter 10 – Patterns of Inheritance How will the offspring inherit these traits & what will they look like? Brown X Blonde Chapter 10 – Patterns of Inheritance How will two traits be transmitted from parents to offspring? Two-trait cross or dihybrid cross – Utilization of Punnett Square Chapter 10 – Patterns of Inheritance How will two traits be transmitted from parents to offspring? Two-trait cross or dihybrid cross – Utilization of Punnett Square Chapter 10 – Patterns of Inheritance Would you want to use a Punnett Square for any crosses involving 3 or more traits? The Product Rule Chapter 10 – Patterns of Inheritance Product Rule Problem #2 Chapter 10 – Patterns of Inheritance Genetic disorders Autosomal dominant disorders Homozygous dominant or heterozygous individuals will have the disorder Huntington disease Degenerative neurological disorder = brain cells die prematurely 1 in 20,000 individuals Onset = middle age Death = within 10 to 15 years Genetic screening = available but would you want it! Chapter 10 – Patterns of Inheritance Genetic disorders Autosomal recessive disorders Homozygous recessive individuals will have the disorder Cystic fibrosis 1 in 20 Caucasians are carriers 1 in 2500 children born will have the disorder Produces thick mucus in the lungs & pancreas, preventing proper functioning of these organs Chapter 10 – Patterns of Inheritance Beyond Mendel’s Laws Polygenic Inheritance When one trait is governed by two or more sets of alleles Skin color as an example The number of pairs of alleles is not known but a simple two pair example can give light on how skin color variability can be attained. Just count the total number of dominant alleles in each of the phenotypes below Phenotype Genotype Very Dark AABB Dark AABb or AaBB Medium Brown AaBb or AAbb or aaBB Light Aabb or aaBb Very Light aabb Chapter 10 – Patterns of Inheritance Multiple Alleles & Degrees of dominance ABO Blood Type How many different blood types are there (disregard + and -)? 4, and they are A, B, AB, & O How do you type blood? Blood type is determined by the presence or absence of glycoproteins embedded in your red blood cell’s membrane Thus if you are: Type A blood you have type A glycoprotein Type B blood you have type B glycoprotein Type AB blood you have both glycoproteins Type O blood you have neither glycoprotein What are the alleles that determine blood type & what genotypes give the above mentioned phenotypes? Chapter 10 – Patterns of Inheritance Alleles IA, IB, i Phenotypes Genotypes A IA IA or IA i B IB IB or IB i AB IA IB O ii What is the relationship between the IA allele and the i allele? What is the relationship between the IB allele and the i allele? Dominant vs. Recessive What is the relationship between the IA and IB alleles? Co-dominance = both alleles within the allele pair are expressed equally Chapter 10 – Patterns of Inheritance Incomplete Dominance The heterozygote condition gives an intermediate phenotype Sickle cell anemia A genetic blood disorder characterized by sickle shaped red blood cells, which result in insufficient delivery of oxygen to the tissues The cells sickle due to a defective hemoglobin gene Hemoglobin is a protein within red blood cells which binds & delivers oxygen to the tissues HbA = normal hemoglobin allele HbS = sickle cell hemoglobin allele Phenotype Genotype Normal HbA HbA Sickle cell trait HbA HbS Sickel cell anemia HbS HbS Chapter 10 – Patterns of Inheritance How does the sickle cell trait fall in between the normal and sickle cell anemia phenotypes? The sickle cell traits red blood cells look normal, but can sickle if the individuals become dehydrated or suffer mild oxygen deprivation The neatest phenotypic difference is their resistance to malaria In high malaria areas of Africa: Sickle cell amenia babies die from sickle cell Normal individuals risk dying from malaria infection Sickle cell trait individuals are immune from contracting the malaria parasite and won’t die from sickle cell Chapter 10 – Patterns of Inheritance Summary of Dominant vs. Recessive, Co-dominance, & Incomplete dominance Let’s use a simple color analogy (Red allele vs. White allele) – symbols for each of the above relationships are different, but the key thing to remember is the homozygous condition vs. the heterozygous condition Phenotype Genotype Dominant Co-dominance Incomplete dominance Homozygous red allele Red Red Red Homozygous white allele White White White Heterozygous Red & White Pink Red The heterozygous condition is the only place that the co-dominant or incomplete dominant phenotype will express itself Chapter 10 – Patterns of Inheritance PRACTICE QUESTIONS 1. What are the laws of segregation & independent assortment? 2. What is a gene? What is an allele? 3. Differentiate between Dominance, Co-dominance, & Incomplete dominance 4. Why is blood typing an example of a multiple allele system? 5. Given that Tom is BB & Mary is bb, where B = big nosed and b = small nosed. What would be the phenotypes of their children & in what percentage? 6. Given that Bob is Bb and Diane is Bb, what would be the phenotypes of their children & in what percentage? 7. What kind of genotypes would Alan’s sperm have if he were BbTt? 8. If he married Sue who had a genotype of Bbtt, how many of their offspring would be bbtt? BbTt? BbTT? 9. Given that Tom is heterozygous for brown eyes & Mary is homozygous for brown eyes, if all of their children are brown eyed what is the relationship between the brown eye allele & the blue eye allele?
