Topic 5: Genetics Heredity is the study of inheritance which is controlled by our genes and genes...
-
Upload
esmond-pitts -
Category
Documents
-
view
219 -
download
2
Transcript of Topic 5: Genetics Heredity is the study of inheritance which is controlled by our genes and genes...
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
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 parentEach 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 MendelFather of Modern Genetics
and Austrian MonkStudied 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 results4.Answer the question!
T = Tall
t = short
TT x tt
T T t tx
• Before we use a Punnett square to predict outcomes of Mendel’s cross, we must define the following:
Parents referred to as P1 (P)
First generation referred to as F1 (F=Filial)
Second generation referred to as F2
Tt Tt
Tt 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
TT
Offspring
Gametes
100% Tall with a genotype of Tt
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 (F1)
generation are bred (Tt x Tt)?tT
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
tT
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
Pp Pp
pp 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 haira = 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
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
1 2
1 2 3 4 5
321
76
4 5 6
1. 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.
Dd dd
dd Dd Dd dd dd Dd dd
dd dd dd Dd dd dd
Legend: D = No dimplesd = dimples
6. What is the chance that the next child of individuals II-1 and II-2 will have dimples?
d Dd
Ddd
D ddd
ddThe chance that their next child will
have dimples is 50%.
Legend: D = No dimplesd = 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
ddd
d ddd
dd
Legend: D = No dimplesd = 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 RatioAll Tall
Genotypic RatioAll Tt
Tt Tt
Tt Tt
No shortoffspring
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
tt tt
Unknown Tall plant (TT or Tt) x short plant (tt)
Unknown is likely Tt to produce both short AND tall
offspring
Phenotypic Ratio2 Tall : 2 short
Genotypic Ratio2 Tt : 2 tt
Some shortoffspring
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 chromosomeonly 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. XRXR
Mostlymales
Mostlyfemales
Sex-linked alleles are symbolized by superscripts attached to the X or the Y
chromosome
Females Males
XRXR (red)
XRXr (red)
XrXr (white)
XRY (red)
XrY (white)
First Cross
All offspring have red eyes, inherited from mom
Xr XR Xr
XRYY
XRLegend: R = red r = white
Parents:XRXR x XrY
GametesXR, XR and Xr, Y
Homozygous dominant red eyed female and a white eyed male
XR
XR Xr
XRY
XR XR XR
XRYY
XR Xr
XR Xr
XrY
Second Generation (F1 Cross)
Legend: R = red r = white
Parents:XRXr x XRY
GametesXR, Xr and XR, 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 XR 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 XR allele
Conclusions
Cross #1 Cross #2
Xr XR Xr
XRYY
XR Xr
Xr Xr
XrY
Legend: R = red r = white
Parents:XRXr x XrY
GametesXR, Xr and Xr, 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 (Xr) 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?
Xc XC Xc
XCYY
XC Xc
Xc Xc
XcY
Legend: C = normal c = colourblindParents:
XCXc x XcY
Gametes
XC, Xc and Xc, 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
XcXcNormal male
XCY
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
(Xc) 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 Inheritance Affected 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
XaXa and XaY
XAXA, XAXa and XAY
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)
XA = affected
Xa = 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)
XA = normal
Xa = 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
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
AaAa
Aa
AaAa Aa
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
XaY XaY
XaXa
XaY