Topic 5: Genetics Heredity is the study of inheritance which is controlled by our genes and genes are found on chromosomes Each person has 2 genes (alleles) for each trait –one from each parent

What patterns do you notice in the table?

What is a gene pool? All the genes in a given population Terminology you need to know

Allele- alternate forms of a gene controlling a characteristic. Alleles are found in the same position (loci) of homologous chromosomes Dominant allele: masks the effect of the recessive allele. Can be expressed in heterozygous form. Represented by an upper case letter: Recessive allele: masked by the effect of the dominant allele. Can only be expressed in homozygous form. Represented by a lower case letter: We inherit one allele from each parent Each trait is controlled by at least 2 alleles Ex. The gene is tongue rolling, the alleles can be for tongue roller or non tongue roller T t T t

Do you have hitchhikers thumb? If you do, you have the two recessive alleles (tt), one from mom, the other from dad If you don’t, you have at least one dominant allele (Tt or TT), the dominant allele could be from mom or dad Page 13 of the data booklet Do you have a hitchhiker’s thumb?

More terminology Genotype – alleles that make up a trait (must be at least 2 letters) E.g. Genotype for tongue rolling could be Tt or TT, non-tongue rollers have a tt genotype Phenotype – observable traits of an organism (physical appearance) dependent on its genotype, but can be affected by our environment »ex: freckles or no freckles Homozygous dominant – genotype with 2 dominant alleles Homozygous recessive - genotype with 2 recessive alleles Heterozygous – genotype with 1 dominant and 1 recessive allele This individual is a carrier for the recessive allele tt Tt TT

Gregor Mendel Father of Modern Genetics and Austrian Monk Studied and breed pea plants

1.They can self-fertilize and/or cross-fertilize  breeding easy to control 2.They have several traits that are expressed in two distinct forms– –different coloured seeds (yellow vs. green), –size of plants (tall vs. short), position of flowers etc. Why garden peas?

What is the phenotype of the following: »RR? »Yy? »gg? Pea traits that Mendel studied Round seed (homozygous dominant) Yellow seed (heterozygous) Yellow pod (homozygous recessive)

Summary of Mendel’s Cross Mendel initially crossed a pure tall and pure short plant together and noticed that all offspring are tall He then took 2 of the offspring and breed them and noticed that ~75% of the offspring were tall and ~25% were short He noticed the same results every time he performed the experiments We can now use Punnett Squares to predict the outcomes of different mating's

Predicting the outcome of a cross? Steps to a Punnett Square: 1.Write down a legend of the letters that you will use to indicate dominant and recessive alleles 1.Write down the genotypes for the parents (write down the given and leave blanks for unknown) 2.Show the different gametes that each parent will produce (circle them!!) 3.Use a Punnett Square to show the results 4.Answer the question! T = Tall t = short TT x tt T T t t x Before we use a Punnett square to predict outcomes of Mendel’s cross, we must define the following: Parents referred to as P 1 (P) First generation referred to as F 1 (F=Filial) Second generation referred to as F 2

Tt Cross 1: Pure tall x Pure short (make sure you write all of this down) What percentage of offspring are tall? What is their genotype? 1. Legend: 2. Genotype of parents: 3. Gametes: T = Tall t = short TT x tt T T t t t t T T Offspring Gametes 100% Tall with a genotype of Tt

TT Tt tt What is the probability of tall offspring? What is their genotype? Tall offspring: 0.75 or 75% Genotype: TT and Tt What happens when offspring from the first cross (F 1 ) generation are bred (Tt x Tt)? t T t T The dominant trait usually shows up more often as it is produced by 2 possible genotypes, Tt or TT 1. Legend: 2. Genotype of parents: 3. Gametes: T = Tall t = short Tt x Tt T t T t

A Punnett square links together what we learned about mitosis, meiosis, haploid and diploid. Label the picture to the right to see the connection

Tt tt Tt tt 1. Legend: 2. Genotype of parents: 3. Gametes: T = Tall t = short Tt x tt T t t t t t t T The probability of having a short offspring is 50% What is the probability of a short offspring, from a cross between a heterozygous tall plant and a homozygous recessive short plant?

Another example… Photo Sensitive Sneezing (sneezing when suddenly exposed to bright light) is a dominant trait. A photo sensitive man (heterozygous) marries a normal woman. What is the probability of a normal girl? How about a normal boy? 1. Legend: P = Photo sensitive (dominant) p = normal (recessive) p p P p PpPp PpPp pp Probability of normal: 50% Probability of a girl: 50% Probability of a normal girl: 0.5 x 0.5 = 0.25 = 25% 2. Genotype of parents: Pp x pp 3. Gametes: P, p and p, p

Pedigree Charts A pedigree chart shows how traits are passed on in a family (similar to a family tree) This info is in your data booklet on page 13

The circle represents a __________ The square represents a __________ Shaded shapes represent family members who express the trait or are affected (for example, dimples). In this example, the ________has dimples. A horizontal line is used to connect __________. (mating) Vertical lines connect parents and ___________. The oldest children are placed on the ________ and the youngest on the ___________. In this example, there are three children — two females and a male. The ________ child has no dimples. Pedigrees female. male. two parents right left children male parents children female

Your turn: Assume that the trait for straight hair is recessive. Write a legend and then determine the genotype and phenotype of all individuals Is it possible that for a recessive trait, 2 affected individuals have a child without the trait? So, if the straight hair trait is not inherited though a recessive allele, how is it passed on? Legend: A = curly hair a = straight hair aa, straight hair phenotype aa, straight hair phenotype Aa, curly hair phenotype No, where does the dominant allele come from? The allele for straight hair is dominant rather than recessive!

Your turn: Assume that the trait for brown eyes is dominant. Write a legend and then determine the genotype and phenotype of all individuals Is it possible that for a dominant trait, 2 affected individuals have a child without the trait? How can you explain this? Legend: A = Brown eyes a = not brown eyes A_, brown eyes phenotype aa, not brown eyes phenotype The parents are both carriers (heterozygous, Aa). We can go back and fill in the genotypes of the parents Yes

II I III Label the generations and each member of the generations.

2. How many children did I-1 and I-2 have? _________________ 4 children 3. What was their sex? ______________________________Female, female, male, female 4. Which one is the oldest child? _______ II-1

5. Assuming that having dimples is the recessive trait and no dimples is the dominant trait. Write the genotypes for all the individuals on the pedigree. Dddd Dd dd Dddd Dd dd Legend: D = No dimples d = dimples

6. What is the chance that the next child of individuals II-1 and II-2 will have dimples? d Dd d Dd dd The chance that their next child will have dimples is 50%. Legend: D = No dimples d = dimples Genotype of parents: Dd x dd Gametes: D, d and d, d

7. What is the chance that the next child of II-4 and II-5’s will be a boy with dimples? d dd d dd Legend: D = No dimples d = dimples Genotype of parents: dd x dd Gametes: d, d and d, d Probability of dimples: 100% (1.0) Probability of a boy: 50% (0.5) Probability of a boy with dimples: 1.0 x 0.5 = 0.5 = 50%

The genotype of individual II-4 is _____________ A person represented on the pedigree chart who has a homozygous genotype is ___________ What are the chances that individual II-3 and II-4 have another child who will have PKU? _____________ Can 2 unaffected individuals have an affected child? How? Do all individuals of affected parents also have to be affected? Why? Pp II-3 or III-2 and III-3 50% Yes, they only have recessive alleles Yes, if they are both carriers

Test Cross Do you have mid-digit hair? –If you do not, you have the homozygous recessive genotype (hh) –If you do, you have at least one dominant gene (H_) But how do you know if you are Hh or HH?!? A test cross is used to determine the genotype (HH or Hh) of an individual who expresses the dominant characteristic What exactly is a test cross? To perform a test cross, we will cross the unknown plant (TT or Tt) with a homozygous recessive (tt) plant and examine the offspring produced

Testcross with monohybrids

Test Cross #1 Phenotypic Ratio All Tall Genotypic Ratio All Tt Tt No short offspring Unknown Tall plant (TT or Tt) x short plant (tt) Unknown is likely TT to produce all tall offspring t ? t ?

Test Cross #2 Tt tt Unknown Tall plant (TT or Tt) x short plant (tt) Unknown is likely Tt to produce both short AND tall offspring Phenotypic Ratio 2 Tall : 2 short Genotypic Ratio 2 Tt : 2 tt Some short offspring t ? t ?

Gender and Inheritance While working with fruit flies (Drosophila melanogaster), Thomas Hunt Morgan, an American Geneticist noticed that there were distinct gender differences in a few things: –Size –Eye colour female male

Sex-linked inheritance Traits coded by genes located on the sex chromosomes are known as sex linked traits Male sex chromosomes: XY Female sex chromosomes: XX Sex-linked traits are almost always on the X chromosome The Y chromosome is used for sex determination Since males only have 1 X chromosome, they cannot be heterozygous or carriers for sex linked traits Traits controlled by genes carried on the Y chromosome only affect males and are known as Y-linked

Eye Color In Drosophila To support the idea that eye color is sex linked, let’s assume the genes controlling eye colour in fruit flies be on the X chromosome Let R = red eyes r = white eyes The Y chromosome does not carry gene for eye color Write all the possible genotypes and phenotypes for eye colour Ex. X R X R Mostly males Mostly females Sex-linked alleles are symbolized by superscripts attached to the X or the Y chromosome FemalesMales X R X R (red) X R X r (red) X r X r (white) X R Y (red) X r Y (white)

First Cross All offspring have red eyes, inherited from mom XrXr XR XrXR Xr XRYXRY Y XRXR Legend: R = red r = white Parents: X R X R x X r Y Gametes X R, X R and X r, Y Homozygous dominant red eyed female and a white eyed male XRXR XR XrXR Xr XRYXRY

XRXR XR XRXR XR XRYXRY Y XRXR XrXr XR XrXR Xr XrYXrY Second Generation (F1 Cross) Legend: R = red r = white Parents: X R X r x X R Y Gametes X R, X r and X R, Y All female offspring have red eyes, one male offspring has red eyes, the other white eyes Heterozygous red eyed female and a red eyed male

First cross: No white eyed female offspring are possible since the mom passed on the X R allele to all her children Second cross: No white eyed female offspring are possible since daughters inherit their X chromosomes from their fathers, whom all had the X R allele Conclusions Cross #1Cross #2

XrXr XR XrXR Xr XRYXRY Y XRXR XrXr X r XrYXrY Legend: R = red r = white Parents: X R X r x X r Y Gametes X R, X r and X r, Y The chances of having a son with white eyes is 25%. Practice Problem: Cross a red-eyed female (heterozygous) with a white-eyed male. What are the chances they will have a white-eyed son?

Examples of sex-linked recessive traits in humans Sex linked recessive traits (X r ) occur more often in males than females Why do you think this is true? Males only have one X chromosome so they cannot be carriers, they either have the trait or not Examples of sex-linked recessive inheritance are: Red-green color blindness Hemophilia - Trouble clotting blood caused by the lack of a blood protein, called Factor VIII, found on the X Chromosome Can you identify these 2 numbers?

XcXc XC XcXC Xc XCYXCY Y XCXC XcXc X c XcYXcY Legend: C = normal c = colourblind Parents: X C X c x X c Y Gametes X C, X c and X c, Y What is the probability that their first child will be a normal son? Practice Problem: Colour-blindness is a sex-linked recessive disorder. A man with color-blindness is married to a woman who is a carrier of the disorder. The chances of having a colourblind son is 25%.

Can boys ever inherit an X linked trait from their dad? Why or why not? Can a colourblind mother have a normal vision son? The following is a pedigree for a family in which colour-blindness has been observed. What is the genotype and phenotype of individual’s I-1 and I-2? Write them on the pedigree What is the probability that individual’s I-1 and I-2 have a daughter that is colour blind? Colourblind female X c Normal male X C Y Do a Punnett Square to answer this. 0%, since the father has normal vision No. Boys inherit the Y chromosome from dad No. Since she only has affected alleles to pass on (X c ) to all her children

Sometimes in science we are given a pedigree but we don’t know if the trait is inherited through an autosome or on a sex chromosome. To predict the probability (chances) of inheriting a trait we use a few simple tools to first determine the mode of inheritance Putting it all together - Modes of Inheritance Is brachydactyly (short fingers and toes) a dominant or recessive trait? Is the gene found on an X chromosome?

We have now studies 5 different modes of inheritance. Mode of InheritanceAffected Genotype(s) Y-linked Autosomal recessive Autosomal dominant X-linked recessive X-linked dominant Use A and a as the alleles Only males are affected Traits are inherited on autosomes (chromosomes 1-22) Traits are inherited on the X chromosome only XY (affected gene found on the Y chromosome) Aa (Aa are carriers) AA and Aa X a X a and X a Y X A X A, X A X a and X A Y

Steps in determining the mode of inheritance Only boys are affected and an affected male must have all affected sons and an affected father If NOT, then we move on To determine how a trait is inherited, follow these steps: Step 1: Is the trait Y-linked?

Autosomal X- Linked (XD) If Yes Affected males have an affected mom and affected daughters If a trait is not X-linked dominant (XD), then we can say it is autosomal Dominant Affected individuals have affected parents Step 2: Is the trait dominant or recessive?

Autosomal X- Linked If Yes More boys affected. Affected girls have an affected father and an affected sons If a trait is not X-linked recessive (XR), then we can say it is autosomal Recessive Affected individual has unaffected parents Click to watch a tutorial

A famous pedigree… Queen Victoria was a carrier for hemophilia (an X-linked recessive disorder) Check to see if the rules work!

1. Y XD AD XR AR Legend: A = a = Y: only males affected, not true (I-1) Characteristics to go through: XD or AD: affected individuals have an affected parent (true) XD: affected males have an affected mom and affected daughters (not true, II-3 is a male, but not all his daughters are affected) affected normal

Y XD AD XR AR Legend: A = a = Y: only males affected, not true (I-1) Characteristics to go through: XD or AD: affected individuals have an affected parent (not true, IV-1 has unaffected parents ) XR: more males affected and affected females have an affected dad and affected sons (not true, more females affected and I-1 is a female, but not all his sons are affected) Normal affected

Y XD AD XR AR Legend: A = a = Y: only males affected, not true (I-1) Characteristics to go through: XD or AD: affected individuals have an affected parent (true) XD: affected males have an affected mom and affected daughters (true, II-3 is a male, his mom is affected and so are all his daughters) X A = affected X a = normal 3.

Y XD AD XR AR Legend: A = a = Y: only males affected, not true (I-1) Characteristics to go through: XD or AD: affected individuals have an affected parent (not true, III-1 has unaffected parents ) 4. XR: more males affected and affected females have an affected dad and affected sons (true, 5 males affected, I-1 is a female and her sons are affected and IV-4 has an affected father) X A = normal X a = affected

Tay Sachs Disease What are the genotypes of all affected individuals? What is the mode of inheritance? Tay Sachs is likely to be autosomal recessive. It skip the first generation parents. It cannot be Y-linked or X-linked recessive because II-1 is an affected girl and does not have an affected father It cannot be dominant because every affected individual must have at least one affected parent. Click to watch a tutorial aa

The trait is likely to be autosomal dominant. It cannot be Y-linked because II-1 is a girl and affected. It cannot be X- linked dominant because affected males do not have all affected daughters It cannot be X- linked recessive because affected moms do not have all affected sons Aa

The trait is likely to be x-linked recessive It cannot be Y-linked because III-3 is a girl and affected. It cannot be X- linked dominant because affected males do not have all affected daughters It cannot be autosomal dominant because I-1 and I-2 are not affected but they have affected children XaYXaY XaYXaY XaXaXaXa XaYXaY