Genetics Chapter 10/12-ish
Learning Goals For Biweekly Quiz #7 You will be able to explain how offspring receive genes from their parents You will be able to calculate probabilities of simple Mendelian traits using Punnett Squares For the Final You will be able to define and explain the 7 different patterns of inheritance You will be able to provide examples of how humans show the 7 patterns of inheritance
Heredity The first scientists to study the laws of heredity had some difficult initial problems to solve Two parents have to contribute equally to make one child Sometimes offspring show similar traits to their parents while in other ways they show traits that don t appear related to their parents in any way Mixed breeds: two different species can sometimes produce offspring Laws of heredity must explain how two parents can allow their traits to mix with each other and make a child (the process is stable, with chaos).
Mendel Gregor Mendel was an Austrian monk who performed the first and most important experiments with genetics Mendel was able to use gametes from pea pods to form special zygotes, and use these zygotes to study heredity Gametes are male and female sex cells. In humans, gametes are sperm and eggs Zygotes are fertilized cells that form when gametes unite with each other
Female Male
Mendel s Experiment Mendel decided to study only one trait at a time. The first trait he chose was tall plants vs short plants. He took male pollen from tall plants and crossed (artificially mated) them with the female parts of short plants. Months later, when he had grown more than 1000 plants, all of the plants were tall and none were short.
Mendel s Experiment Mendel wondered what happened to all the short plants. So he crossed all of the offspring from the first experiment with each other. In the second set of offspring, Mendel observed 787 tall plants and 277 short plants He realized that the ratio of tall plants to short plants was surprisingly close to 3:1 (74% tall, 26% short).
Mendel s experiment Mendel had some problems to work out 1 st, how come when he crossed tall plants and short plants only tall ones were born? 2 nd, how come when he crossed only tall plants that both tall and short plants were born? One trait of the parent generation would disappear in the first generation, but reappear in the second generation. What was going on? Mendel repeated the experiments with different traits, but each time he got the same results.
Seed shape
Generations P generation Parent generation. The first cross you start with. (Your parents) F1 generation The offspring of the parent generation (You) F2 generation The offspring of the F1 generation (Your future kids)
Chromosomes Each chromosome in an organism contains a portion of the blueprint necessary to build that organism These chromosomes are made of sequences of DNA called genes. Each gene codes for a specific trait in an organism.
Chromosomes Mendel concluded that each trait for an organism must be controlled by two genes, one from the father s chromosome and one from the mother s chromosome Each gene can have more than one option (called alleles) for what it will code for. The different options for each gene are alleles Example: Eye color alleles are Brown and Blue;
Dominance If each gene has two alleles, why will only one show up? Mendel realized that one allele for each gene must be a dominant one. The dominant allele is the allele for the trait that shows up. The recessive allele is the allele for the trait that disappeared In pea plants, the allele for tall plants is dominant and the allele for short pea plants is recessive.
Genotypes and Phenotypes A genotype is the combination of alleles that an organism has Each allele is assigned a letter. Capital letters mean dominant, lowercase mean recessive T=tall allele. t=short allele. Tt, TT, tt are all examples of genotypes. A phenotype is the trait that actually appears TT, Tt genotypes will each have a tall phenotype tt genotypes will have short phenotype
Genotypes and Phenotypes Phenotypes do not take into account that an organism may have an allele that isn t showing up Example: A plant with a Tt genotype would have a tall phenotype, even though one of the alleles codes for short. When both alleles are the same, we call it homozygous. When they are different, we call it heterozygous. TT, tt = homozygous Tt = heterozygous
Punnett Squares When you understand the vocabulary and the process behind genetics, you re not only able to calculate what genes an organism likely has, but what gene s it s offspring are likely to have In other words, you can calculate the likelihood of what your kids will look like. We use a tool called Punnett Squares to calculate these odds.
Punnett Squares Monohybrid Crosses Use a monohybrid cross if you want to calculate the odds of only one gene On the top, put the two alleles of one parent On the left side, put the two alleles of the other parent Then fill in the boxes according to the allele from each parent T t T TT Tt t Tt tt
Punnett Squares Monohybrid Crosses Each of the squares represent the potential sequence of genes in the offspring of the parents Once the squares are filled in, you can start calculating ratios Genotypic Ratios 1:2:1 TT : Tt : tt Phenotypic Ratio 3:1 Tall : Short T t T TT Tt t Tt tt
Punnett Squares Dihybrid Crosses What if you want to calculate the possibility of two genes? Dihybrid crosses are larger, but the overall concept is the same. For this example, we will use Mendel s experiment: The shape of peas AND the color of peas Round (R) is dominant over Wrinkled (r) Yellow (Y) is dominant over Green (y)
Punnett Squares First, find the four possible allele combinations for each parent. Assign one combo for each row and column. Round = R; Wrinkled =r Yellow = Y; Green = y RY Ry RY Ry ry ry Fill in the squares as with the monohybrid cross ry ry
Punnett Squares When you calculate the different squares, you end up with a 9:3:3:1 ratio
Pattern #1: Simple Heredity Patterns of heredity are the rules that the gene or genes for a specific trait follow. As it turns out, Mendel only discovered the simplest of these heredity patterns Simple Mendelian heredity follows three rules Rule #1: There is only one gene for this trait. Rule #2: This trait has only two alleles Rule #3: One allele is dominant over the other allele. The rest of the following inheritance patterns break at least one of these rules.
Pattern #2: Incomplete Dominance In simple heredity, heterozygotes will display the trait of the dominant allele because it is dominant. With incomplete dominance, heterozygotes will instead show an intermediate, mixture phenotype. In incomplete dominance, there are no dominant traits. Each trait is a blend of the alleles that an organism has.
Pattern #2: Incomplete Dominance In Punnett squares, you can t use capital and lowercase letters for incomplete dominance. Instead, you use two capital letters, one of which has an apostrophe Red allele in snapdragons: White allele in snapdragons: R With incomplete dominance, three different genotypes will result in three different phenotypes All other rules of genetics and reading Punnett squares apply R
Pattern #3: Codominance Codominance is when one allele is not dominant over the other, but both are equally dominant. If the organism is homozygous, the organism will have only one phenotype. If the organism is a heterozygote, they display both phenotypes at once rather than a mixture of phenotypes In Punnett squares, both codominant alleles are uppercase and given a different letter.
Black-feathered chicken: White-feathered chicken: Checkered chicken: BB WW BW
Pattern #4: Multiple Alleles Multiple alleles are when a gene has more than two possibilities There could be between 3 and 100 different alleles Multiple allele traits follow all other patterns of dominant and recessive. Each organism still only has two alleles within their DNA code (one from each parent), but the number of different combinations of alleles could be numerous.
Pattern #4: Multiple Alleles In Punnett squares, for a multiple allele trait, each allele has a single symbol followed by a superscript Y=Yellow Lab; Y b =Black Lab; Y c =Chocolate Lab The only for sure way to read the dominance is if a key is provided for you. Yellow>Black>Chocolate
Pattern #5: Sex-Linked Traits Every human has 22 pairs of chromosomes that are exactly alike, male or female. The final pair of chromosomes is different for males and females. Females have two X chromosomes, males have an X and a Y chromosome. The X and Y chromosomes each contain different genes
Pattern #5: Sex-Linked Traits A female will not have any gene located on a Y chromosome What if a gene is located on the X chromosome? Males only have one X chromosome. They will only have one allele for that trait, dominant or recessive. Females have two X chromosomes, so they will have two alleles for the trait. Because of this, it is harder for females to have a recessive X-linked phenotype than it is for males.
In Punnett squares, sex-linked traits are expressed as an X or a Y. There is also a superscript to describe which allele is being represented Fruit flies: X R =Red eyes. X r =white eyes. Y=Y chromosome, so no gene is present on the chromosome
Pattern #6: Sex-Determined Traits Sometimes organisms have a gene for a specific trait, but the trait is not expressed because of the organism s gender. A sex-determined trait is when a trait only appears in a certain gender Hormones produced by other genes block sexdetermined genes from expressing in an organism Reason: Parents of one gender may not express the same traits as children of the opposite gender. However, parents still need to pass all necessary genes to their child regardless of age.
Pattern #7: Polygenic Inheritance Polygenic (Poly=many, genic=genes) is when a trait is controlled by more than one gene. Each gene may have 2 or more than 2 alleles Polygenic inheritance resembles incomplete dominance. Rather than one allele being dominant, all the alleles in all the genes contribute an equal portion towards the phenotype
Pattern #7: Polygenic Inheritance Instead of Punnett squares, you write each gene in a list. Each gene is given a different letter Each trait is given a different form of the letter A, B, C, D could all be the genes Uppercase and lowercase, or superscripts, could be alleles A Y A R B Y B Y C R C R D Y D R Y=Yellow corn, R=Red Corn. This genotype has 4 Yellow alleles and 4 Red alleles among all the genes. Look for keys on polygenic inheritance as well.
Simple Inheritance in Humans We ve talked about many of these already Earlobes Ear wax We will now go through each of these patterns of inheritance and see how they apply to humans
Incomplete dominance in Humans Incomplete dominance is when the heterozygous phenotype is a mix of the two homozygote phenotypes. For humans, an example of this is Hair style Alleles: H=Straight hair, H = Curly Hair Genotypes HH=Straight Hair HH =Wavy Hair H H =Curly Hair
Codominance in Humans Codominance is when heterozygote organisms have a phenotype that equally shows both traits. An example of this for humans is sickle-cell anemia, when blood cells form sickle shapes instead of round shapes. Alleles: R=Round, S=Sickle Cell Genotypes RR=All round blood cells RS=1/2 round blood cells, ½ sickle blood cells SS=All sickle blood cells
Multiple Alleles in Humans Multiple alleles are when there are 3 or more possible alleles in a gene. An important trait for humans that expresses multiple allele inheritance is blood type Your immune system must recognize the difference between foreign substances and your own blood To do this, your blood has specific proteins called antigens on its plasma membrane. Your immune system recognizes these proteins and knows that the blood cell belongs to you and isn t an intruder
Multiple Alleles in Humans The different antigens are labeled A and B Alleles: I A =A-Type Blood; I B =B-Type Blood; i=neither type There are 4 possible phenotypes of blood, arising from 6 possible genotypes Genotype Antigens Present Phenotype (Blood Type) I A I A (AA); I A i (AO) A-Type only Type A I B I B (BB); I B i (BO) B-Type only Type B I A I B (AB) Both A and B Types Type AB ii (OO) Neither A or B Types Type O
Multiple Alleles in Humans It is important for you to know what your blood type is BEFORE you get a blood transfusion If you get blood with a different protein than what your immune system is used to, it will attack the blood This results in blood clots and, usually, is deadly (Because O type blood has NO proteins on it, your cells won t recognize the WRONG proteins.) If you have this blood type Type A Type B Type AB Type O You can receive these blood types Type A or Type O Type B or Type O Type A or Type B or Type O Type O only
Sex-Linked Traits in Humans Females have two X chromosomes. Mothers can only donate an X chromosome to their offspring Males have both an X and Y chromosome. Whichever chromosome a father donates determines his offspring s gender A male has an easier time of having a phenotype from a recessive sex-linked trait than a female Females need to receive two recessive alleles to have the recessive trait. Males can only receive one allele. A father cannot pass a sex-linked Y-chromosome trait to his daughter OR an X-chromosome trait to his son.
Recessive sex-linked trait
Sex-Determined Trait in Humans A sex-determined trait is a gene that will only be expressed depending on your sex Both men and women produce testosterone and estrogen. Around puberty, the body begins to produce higher amounts of one or the other Your sex determines which structures your body will form (testes or ovaries), and these structures produce high quantities of testosterone and estrogen, respectively Therefore, even though you have the gene to produce both hormones, your sex decides which you will produce more of.
Polygenic Inheritance in Humans Polygenic Inheritance is when a trait is controlled by multiple genes. In humans, skin color is controlled by between four and six different genes. Each of these genes will have two alleles Alleles for some genes: Yellow and Red Alleles for other genes: Light to Dark
Polygenic Inheritance in Humans Your skin color is determined by combining the all of the alleles in all of your genes Gene labels: A, B, C, D, E, F Superscript Labels: Y=Yellow, R=Red, L=Light, D=Dark Possible phenotypes: White, Yellow, Brown, Red, Olive, Dark, or a combination of all these Examples of phenotypes A R A L B L B L C L C Y D D D L E L E R F Y F L =2 Yellow, 2 Red, 7, Light, 1 Dark A R A L B D B D C R C R D L D R E L E D F R F R =0 Yellow, 6 Red, 3 Light, 3 Dark
Dolly Wally This is Dolly and Wally. Dolly and Wally have brown hair. Each of them have six genes for hair color, a total of 12 alleles. Each of them also has three alleles for each gene (black, brown, red and blonde). If they have three children, is it possible for these children to have black, red, or blonde hair?
Dolly Wally Holly Their first child is Holly. She has black hair, because she received a total of six black alleles, four brown alleles, and one each of red and blonde.
Dolly Wally Holly Ollie Their second child is Ollie. He s a redhead, because his parents gave him a total of six red alleles, four blonde alleles, and two brown alleles
Dolly Wally Holly Ollie Molly Their third child is Molly. She has blonde hair because her parents gave her six blonde alleles, three brown alleles, two red alleles and only one black allele. Oh what a diverse family of hair color!
Meiosis Explains Mendel Mendel wrote two laws which, now that we know about meiosis, make sense. Law of Independent Assortment, which says that each allele is inherited separately (in other words, there s no rule such as if you have blue eyes you MUST be 5 0 tall.) Crossing over explains this. Law of Segregation, which says that genes can be randomly organized and matched Anaphase II and fertilization both explains this.
Extra Credit Question Quiz #7 You may check your answers with me ahead of time for a yes or no response as many times as you like. Some genes can only be inherited from your mother. HOWEVER, both males and females can have these genes. How is this possible?