Chapter 11
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).
When Charles Darwin wrote Origin of Species he was attempting to explain how species evolve from one generation to the next He had to explain how traits are passed for this to make sense Darwin proposed a blending theory, which says that parent genes (called particles at the time) blend traits to produce an offspring In the meantime an Austrian monk named Gregor Mendel attempted to mathematically explain the change from one generation to the next (Mendel was able to explain how genes pass from parent to offspring even though genes as we know them wouldn t be officially discovered for another 100 years)
Mendel was a mathematician, so he attempted to explain heredity using statistics and data He used the common garden pea for his experiments The pea is easy to cultivate, has a short generation time, and can be self-pollinated or cross-pollinated The first experiment involved crossing (mating) gametes of a tall pea plant with gametes of a short pea plant. Gametes are male and female sex cells (egg/sperm, etc) This first generation is called the P generation Hypothesis: If the blending theory is correct, then all offspring should be medium length because each receive an equal share of genes from their parent The first offspring generation is called the F1 generation
Mendel crossed plants thousands of times to ensure the accuracy of his data When the F1 generation grew, his results were contrary to his hypothesis: 100% of the plants were tall plants. Only one parental trait was passed on. Did the short parental genes disappear? Mendel then crossed members of the F1 generation with each other to produce the F2 generation. The results: 787 tall plants and 277 short plants The ratio was pretty close to 3:1 (74% : 26%) The short trait had disappeared for a generation, but reappeared later
Perhaps, however, this was just a trait with the height of plants Mendel repeated the same experiments over the next few months with the following traits: pea pod shape, seed shape, pod color, flower color, seed color, flower position. Each time, the same results: 100% of one trait in the F1 generation, a 3:1 ratio in the F2. *Note: this ratio represents the simplest type of gene. Only a small percentage of all genes in all organisms are actually this simple and easy to calculate. And Mendel happened to pick those genes. In other words Mendel got really lucky.
After analyzing all possible mathematical explanations for his results, Mendel wrote his first of two laws: The Law of Segregation Each organism has two factors for each trait When gametes form in the organism, each gamete contains only one of the two factors When gametes fertilize, each new organism contains one factor from each parent for each trait
We now know that these factors are the strands of DNA that contain our genes Each gene has a minimum of two possible alleles An allele is an alternate form of the same gene Gene: plant size. Alleles: tall and short One of these genes is a dominant allele and the other is recessive Dominant alleles means the trait they code for will always appear in an organism Recessive alleles can be masked (covered, but not absent) by the dominant allele
Because you receive a set of genes from each parent, eukaryotic organisms all have two alleles for each gene (one from mom, one from dad) The combination of alleles an organism has is called their genotype Each allele in a genotype is given a single-letter label Capital letters are dominant, lower-case are recessive The actual trait that appears in an organism is called a phenotype T=tall plants, t=short plants TT or Tt genotypes = tall phenotypes tt is only genotype that codes for a short phenotype Genotypes with the same allele are called homozygous. Different alleles are called heterozygous
Mendel noticed that in his crosses different combinations of genes always occurred You never ALWAYS had green peas with tall plants. You could also have yellow/tall, green/short, yellow/short This must mean that genes are not connected to each other. This led to Mendel s second law, the Law of Independent Assortment Each gene separates independently from itself Every theoretical combination of alleles is possible within an individual organism Mendelian, or Simple Heredity traits, are traits with the following three rules: the trait is found on only one gene, has only two alleles, and one allele is dominant over the other
There are many traits that break one of the three Mendelian rules Incomplete dominance In incomplete dominance, the heterozygote phenotype actually is a blending of the two alleles Snapdragon flower color is an incomplete dominance trait R=red flowers R =white flowers RR= red flowers R R = white flowers RR = pink flowers Hair style (straight, wavy, curly) is an incomplete dominance trait in humans
Codominance Codominant traits are traits that show both alleles equally in the genotype Dairy cow fur has is a codominant trait B = Black fur W = White fur BB = All black WW = All white BW = Black and White spotted A human example of codominance is sickle blood cells The two traits are round blood cells and sickle-shaped blood cells
Multiple Allele Traits Multiple allele traits are traits that have three or more alleles The alleles all have an order of dominance Labrador fur shows multiple alleles Y=Yellow Y 1 =Black Y 2 =Chocolate YY; YY 1 ; YY 2 = Yellow Y 1 Y 1 ; Y 1 Y 2 = Black Y 2 Y 2 = Chocolate For humans, a multiple allele trait 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. Antigens are glycolipids Your immune system recognizes these proteins and knows that the blood cell belongs to you and isn t an intruder
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 (O) Neither A or B Types Type O
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
Polygenic Traits Polygenic traits follow all normal Mendelian rules, but are combinations of multiple different genes Seed color in wheat: three different genes Human height and skin color: an unknown number of genes, but we think its between 5 and 12. Polygenic traits tend to result in what appears to be multiple or infinite different phenotypes
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!
Epistasis Epistasis is when one gene masks the effect of another gene. Albinism in humans is an example of epistasis Humans have multiple genes for what their skin color will be. The different tones of color are controlled by how much melanin is produced by skin cells The more melanin, the darker the skin They have another gene elsewhere that controls whether or not melanin will be produced If no melanin is produced, then the organism will be an albino and their skin color genes no longer matter
These diseases are recessive alleles that can be passed from parents to offspring Tay-Sachs disease Neurological impairment at 7-8 months old Blindness, seizures, paralysis possible Cystic Fibrosis Mucus forms in the bronchial tubes and prevents lungs from working properly CF children develop more slowly and only live to 20-30 years Phenylketonuria Cannot digest the amino acid phenylalanine Results in severe mental disability
Neurofibromatosis Tumors covering the nerve endings may cause deformations in bone and tissue structure Huntington s Disease Brain cells begin to deteriorate at age 40 Victims lose motor and cognitive function as well You can be tested to see if you have the gene for Huntington s, but there is no cure Question: If these diseases are dominant, how come hardly anyone ever get s them?
Humans have a total of 46 chromosomes (23 from mom, 23 from dad Homologous chromosomes #1-22 are all called autosomes. These chromosomes contain the same genes no matter which parent they came from Not necessarily the same alleles. There is one set of homologous chromosomes that may be different. These are the sex chromosomes Two X chromosomes (Female XX) One X chromosome, one Y chromosome (Male XY) These chromosomes obviously contain genderdetermination genes, but they have other genes as well that don t relate to gender. Any trait controlled by one of these chromosomes is called a sex-linked trait.
Females have two X chromosomes. Therefore, mothers can only donate an X chromosome to their offspring Males have both an X and Y chromosome. If Dad donates an X chromosome, the offspring will be female. If Dad donates a Y chromosome, the offspring will be male. A father cannot pass a sex-linked Y-chromosome trait to his daughter OR an X-chromosome trait to his son.
A female will never show a phenotype from a Y- chromosome s gene X-chromosome sex-linked traits are harder to track Males only have one X chromosome. They will only have one allele for any X-linked trait Dominance/recessiveness doesn t apply. Females have two X chromosomes, so they will have two alleles for the trait. Dominance/recessiveness still applies It is harder for females to have a recessive X-linked phenotype than it is for males. She needs 2, he only needs one Phenotype ratios for sex-linked traits are different depending on the gender of the offspring
Muscular Dystrophy Muscles are weak, to the point where the victim loses almost all use of their muscles Death usually results by age 20 Color Blindness Color-blind people have difficulty distinguishing colors, particularly in the red/green spectrum Hemophilia Hemophilia is an absence of the ability to clot blood. Fragile X syndrome A form of mental disability, but victims are able to live to become a grandfather
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.
Both men and women produce testosterone and estrogen. Around puberty, the body begins to produce higher amounts of one or the other Your gender 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 gender decides which you will produce more of.