Calculating the probability of our genetics Understanding how our genes are passed down And how to calculate the probabilities of our traits.
Leading questions: 1. What do Punnett Squares mean? 2. How do we calculate the probabilities of our traits? 3. What traits will pass down and by how much?
Do you remember these?
What do the y and Y represent and where are they from?
The gene that results in the sex cells are the letters from Punnett Squares. Recall from meiosis A pair of homologous chromosomes (chromosomes of similar traits) are crossed over and separated.
What does that mean? If the red chromosome contains the genes for Brown eyes (B). If yellow contains genes for Blue eyes (b) B b Sister chromatids After crossing over, the genes switch chromosomes with the other. B b B b
What does that mean? At the end of meiosis, all 4 of these sister chromatids will separate (segregate) into 4 sex cells. B B b B b Because the probability of getting B and b is the same, that is why we use Bb! b b B b
When the genes of the homologous chromosomes separate into separate sex cells, Mendel classified this as the Law of Segregation. This will result in a sex cell with traits that are either from your mom (brown eyes) or dad (blue eyes).
We use symbols to represent dominant and recessive genes. The dominant gene is always represented by a capital letter. E.g. A The recessive gene is always represented by the same letter in the lower case. E.g. a To write the combination of a trait, two letters must be used. Ie. AA, aa, Aa
Example Brown hair is a dominant gene Blonde hair is a recessive gene Brown hair s gene will be expressed because it is the dominant gene
Other important terms Alleles - Alternative forms of a gene for a given trait (Blue vs brown eyes) Genotype the alleles possessed by an individual for a specific trait. Eg: BB, Bb, and bb are possible genotypes for eye colour. 2 copies of dominant allele = homozygous dominant genotype (BB). 2 copies of recessive allele = homozygous recessive genotype (bb). 1 copy of each = heterozygous genotype (Bb).
Phenotype the physical appearance of a trait (ie: what the genes actually do to us). Ex. Blue eyes, short, long toe nails, Extra hormones. P for physical traits!
How do we calculate the probability of our genes?
Monohybrid crosses Monohybrid Crosses A cross (combination of alleles) for one trait = a monohybrid cross. Eg: If a heterozygous woman with a widow s peak reproduces with a man with the same genotype, what kind of hairline will their children have? (Note: widow s peak is dominant over straight hairline).
Monohybrid crosses You may also be asked to determine genotypic ratios and phenotypic ratios in the offspring: Genotypic ratios: 1 WW : 2 Ww : 1 ww (or, more simply, 1:2:1) Phenotypic ratios: 3 widow s peak : 1 straight hairline (3:1) W w W WW Ww w Ww ww **Phenotypic ratios can also be written as percentages: 25% chance of straight hairline 75% chance of widow s peak
Test cross What if someone shows the dominant phenotype, but we do not know if they are homozygous dominant or heterozygous? We can do a test cross: Cross the individual with the unknown genotype with one that is homozygous recessive. Why homozygous recessive?
Test cross Example: A kid has brown eyes (dominant trait) but we don t know if he is heterozygous or homozygous dominant. How can we determine his genotype? Use a test cross The kid s genotype could be 2 possibilities. If ALL of the offspring in the test cross show the dominant trait, then the kid s genotype is homozygous dominant. Kid s genotype B b b Bb bb b Bb bb Kid s genotype B B b Bb Bb b Bb Bb
Test cross In Border Collies, black coat (B) is dominant to red coat (b). A breeder has a black male that has won numerous awards. The breeder would like to use the dog for breeding if he is purebred or BB. To learn this information, she testcrosses him with a red female (bb). 1. If the black male is BB, what kind of gamete (sperm) can he produce? B 2. If the red female is bb, what kind of gamete (eggs) can she produce? b
Test cross In Border Collies, black coat (B) is dominant to red coat (b). A breeder has a black male that has won numerous awards. The breeder would like to use the dog for breeding if he is purebred or BB. To learn this information, she testcrosses him with a red female (bb). 2. If any of the puppies are red, what is the father's genotype? Black male could be: BB or Bb Test with bb
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Punnett Squares with Blood types There are 4 main blood types in humans: A, B, AB, and O Blood type A and B are dominant, Type O is recessive. We use I A to indicate a dominant A allele gene and I B for allele B blood. ii represents recessive for O blood.
Punnett Squares with Blood types In order to get type AB blood, both dominant alleles of A and B must be expressed together. They are co-dominant (see later). Ex. I A I B
Blood type Example Could a man with type B blood and a woman with type AB produce a child with type O blood? 2 possibilities for type B blood: I B I B or I B i
Blood type Example Could a man with type B blood and a woman with type AB produce a child with type O blood? Since there s no homozygous recessive (ii), Type O blood is not possible.
Blood type Example 1. What if the mother is type O and the father is A? What would the offspring's blood type be?
Blood type Example 2. A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid?
Sex-linked problems with Punnett Squares Traits controlled by genes on the sex chromosomes are called sex-linked traits. We usually focus on traits controlled by the X chromosome This is because it has many more genes than the Y chromosome!
Sex-linked problems with Punnett Squares Notation for sex-linked genes: Genes carried on the X chromosome: Use the letter X to indicate that the gene is on the X chromosome Use a superscript letter for the trait itself. Eg: X B = dominant gene for normal colour vision X b = recessive gene for colour-blindness
Sex-linked problems with Punnett Squares Examples: Red-green colour-blindness (X-linked) X B = normal; X b = colour-blind Notice how the genotypes are written and what the phenotypes are! Genotype X B X B X B X b X b X b X B Y X b Y Phenotype Female; normal vision Female; normal vision (carrier) Female; colour blind Male; normal vision Male; colour blind
Sex-linked problems with Punnett Squares Note: Because males only have one X chromosome, they cannot be carriers of sex-linked disorders or conditions. They either have the gene and show the condition, or don t have it and don t show the condition. Cannot be a carrier!!! Genotype X B X B X B X b X b X b X B Y X b Y Phenotype Female; normal vision Female; normal vision (carrier) Female; colour blind Male; normal vision Male; colour blind
Sex-linked problems with Punnett Squares Examples cont d: Hemophilia (deficiency in blood clotting factors) Duchenne muscular dystrophy (These are all recessive conditions)
Sex-Linked Inheritance Practice Problems Both the mother and the father of a colorblind male appear to be normal. Colourblindness is recessive. From whom did the son inherit the allele for colorblindness? What are the genotypes of the mother, father, and the son?
Sex-Linked Inheritance Practice Problems A woman is colorblind. What are the chances that her son will be colorblind? If she is married to a man with normal vision, what are the chances that her daughters will be colorblind? Will be carriers?
Sex-Linked Inheritance Practice Problems Both the husband and the wife have normal vision. The wife gives birth to a colorblind daughter. Is it more likely the father had normal vision or was colorblind? What does this lead you to deduce about the girl s parentage?
Sex-Linked Inheritance Practice Problems What is the genotype of a colorblind male with long fingers is s=long fingers? If all his children have normal vision and short fingers, what is the likely genotype of the mother?
Tracing x-linked with pedigree Pedigree Charts! Pedigree Charts tracking x- linked disorders
Tracing x-linked with pedigree My dog s Purebred Pedigree: http://www.k9data.com/pedigr ee.asp?id=116679 Pedigree charts tracks the genetic traits over generations similar to a family tree. Squares indicate MALE Circles indicate FEMALES Pedigree Charts!
Tracing x-linked with pedigree Example 1: Leopold s family (Queen Victoria s son) (a)what is the probability that her other son was hemophilic? (b)what is the probability that her daughter was a carrier? Hemophilic? (c)what is the probability that both children were normal?
Tracing x-linked with pedigree Example 1: Victoria s youngest daughter Beatrice s family (a)looking at the pedigree of the royal family, identify which of Beatrice s children received the hemophilic gene; why can you make this conclusion? (b)notice that Beatrice s daughter, Eugenie, married King Alfonso XIII of Spain and had six children, one of whom was the father of Juan Carlos, the current King of Spain. Would you predict that Juan Carlos was normal, a carrier, or a hemophilic?
Tracing x-linked with pedigree Drawing Example A man and woman marry. They have five children, 2 girls and 3 boys. The mother is a carrier of hemophilia, an X-linked disorder. She passes the gene on to two of the boys who died in childhood and one of the daughters is also a carrier. Both daughters marry men without hemophilia and have 3 children (2 boys and a girl). The carrier daughter has one son with hemophilia. One of the noncarrier daughter s sons marries a woman who is a carrier and they have twin daughters. What is the percent chance that each daughter (twins) will also be a carrier?
Dihybrid Crosses Dihybrid Crosses (2 alleles)!
Dihybrid Crosses Mendel s 2 nd Law is the Law of Independent Assortment: The Law of Independent Assortment is when there are 2 different traits on the homologous chromosome (eye colour and hair colour) Each of these traits will independently separate. For example: Bb (eyes) and Hh (hair) will separate into BH Bh Hb and hb. The important part is that the traits do not stick together
Dihybrid Crosses This leads us to dihybrid crosses Tracking the inheritance of 2 alleles
Dihybrid Crosses Ex. A person who is WwSs (widow s peak and short fingers) and a person who is also WwSs have children: 1. Figure out the gametes for each parent WS, Ws, ws, and ws for both Place these combinations on the edges of a Punnett square 2. Figure out the genotypes of the offspring (use a Punnett square)
From the Punnett square: Phenotypic ratio - 9 widow s peak, short fingers; 3 widow s peak, long fingers; 3 straight hairline, short fingers; 1 straight hairline, long fingers (9:3:3:1)
Dihybrid Crosses To calculate the chances of having two traits, you must multiply the probabilities of having each individual trait and report the result. Eg: From the previous problem, what are the chances of having a child with short fingers and a widow s peak? Chance of short fingers = ¾ Chance of widow s peak = ¾ Chance of having both = ¾ x ¾ = 9/16
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