Theoretical genetics Ms. Jackie. Important definitions Genotype- the alleles possesed by an organism...
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Transcript of Theoretical genetics Ms. Jackie. Important definitions Genotype- the alleles possesed by an organism...
Theoretical genetics
Ms. Jackie
Important definitions
Genotype- the alleles possesed by an organism
Phenotype- all the characteristics of an organism.
Dominant allele- an allele which has the same effect on the phenotype whether it is present in the homozoygous or heterozygous state
Recessive allele- an allele which only has an effect on the phenotype when present in the homozygous state
Important definitions
Codominant alleles- alleles which only has an effect on the phenotype when present in heterozygous but a greater effect in homozygous individuals.
Locus- The particular position of a gene on homologous chromosomes.
Homozygous- two identical alleles of a gene Heterozygous- having two different alleles of a gene Carrier- an individual that has a recessive allele of a
gene that does not have an effect on the phenotype Test cross- testing a suspected heterozygote by
crossing with a known homozygous recessive.
Construction of Punnett grids
Way of finding he expected ratio of the offspring, giving certain parental phenotypes
Punnett grids
Example using characteristics used by Mendel He study the size of
pea plants and found that tall is dominant over short
Lets start with two pure breeding (homozygous) plants with contrasting traits (tall and short)
F1 generation will have the dominant phenotype tall but will be heterozygous.
Tt Tt
Tt Tt
Phenotypes Corresponding genotypes
Tall TT or Tt
Short tt
T T
t
t
Punnett grids
IF the F1 generation self fertilized we will obtained F2:
¾ tall
¼ short
3:1
1TT:2Tt:1tt
TT Tt
Tt tt
T t
T
t
Lets practice
IF a tall garden pea is crossed with a dwarf garden pea, the F1 are all tall. Predict the result of self fertilization of the F1, using a Punnett square.
Pedigree chart
Used to show the inheritance of certain traits over several generations of humans
Example Queen Victoria hemophilic condition Males are normal or affected Females normal or carriers Hemophilia allele is homozygous lethal as well as
sex linked since it is carried on the X chromosome Males can be XHY (normal) or XhY (affected) Females can be XHXH (normal) or XHXh (carrier)
Queen Victoria and hemophilia
Normal male
Affected male
Normal female
Affected female
Victoria Albert
Victoria Edward Alice Alfred Helena Louise Arthur Leopold Beatrice
George V
George VIDuke of Windsor
Elizabeth Margaret
3
3
Irene Frederick Alexandra NicholasII
Alexis
Multiple alleles
Some genes have more than two allelesMultiple alleles can be found in blood
groups
ABO blood groups
Based on 4 different phenotypes (A;B;AB and O)
Caused by different combinations of 3 different alleles
Phenotypes Genotypes
A IAIA or IAi
B IBIB or IBi
AB IAIB
0 ii
ABO blood groups
Using a Punnett Grid work out the possible combinations of blood groups of a male with blood group B and a female with blood group
Using a Punnett Grid explain why a female with blood group O and a male with blood group AB cannot have children with either pf the parents´ blood group.
How sex chromosomes determine gender
Gender determinationDecided by sex chromosomesFemales has two X chromosomesMales have one X and one Y chromosome
Genes in X and Y chromosomes
The X chromosome is relatively large
Y chromosome is much smallerSeveral genes are located on the X
chromosomeSex linked examples
Colorblindnesshemophilia
Few genes on the YHairy ears
Define sex linkage and state two examples
Genes carried on the sex chromosomes, most often on the X chromosome.
Examples Color blindness
Certain colors cannot be distinguished, and is most commonly due to an inherited condition. Red/Green color blindness is by far the most common form, about 99%, and causes problems in distinguishing reds and greens. Another color deficiency Blue/Yellow also exists, but is rare and there is no commonly available test for it.
Depending on just which figures you believe, color blindness seems to occur in about 8% - 12% of males of European origin and about one-half of 1% of females. Total color blindness (seeing in only shades of gray) is extremely rare.
There is no treatment for color blindness, nor is it usually the cause of any significant disability.
Define sex linkage and state two examples
Hemophilia Blood condition in which an essential clotting factor
is either partly or completely missing. This causes a person with hemophilia to bleed for
longer than normal. Cuts and grazes are not great problems as a little
pressure and a plaster are usually enough to stop bleeding.
The main problem is internal bleeding into joints, muscles and soft tissues.
Hemophilia is a lifelong inherited genetic condition, which affects females as carriers and males who inherit the condition.
Human females can be homozygous, hetero with respect to sex linked genes
Human males cannot be heterozygous since they only have one copy of one alleleExample
ColorblindnessExisting alleles XB for normal vision
Xb for colorblindness
A female can be XBXB or XBXb XbXb
A male can be XBY XbY
Human females can be homozygous, hetero with respect to sex linked genes
Hemophilia example
Existing alleles XH for normal Xh for hemophilia
A female can be XHXH or XHXh XhXh (not exist)
A male can be XHY XhY
•Homozygous lethal•Female carriers are heterozygous for X linked recessive alleles
•They will not show the disease but can pass it on to the next generation
Explain that female carriers are heterozygous for X-linked recessive alleles
Heterozygous females for sex linked traitsAre said to be carriersThey will not show the diseaseCan pass the disease to the next generation
Calculate and predict the genotypic and phenotypic ratios of monohybrid crosses
Monohybrid crosses Involves the cross
between two individuals to study the inheritance of one gene
Example Character:
Tongue Rolling Gene:
R Alleles:
R, r Genotypes:
RR, Rr, rr
Phenotypes: rollers, non rollers
Cross: Heterozygous X
Heterozygous Rr X Rr Gametes R, r X R, r Punnett square
RR Rr
Rr rr
R r
R
r
Ratio 3:1
Calculate and predict the genotypic and phenotypic ratios of monohybrid crosses
In your notebooks calculate the genotypic and phenotypic ratios of a cross between a heterozygous tongue roller and a recessive individual for tongue rolling
Calculations and codominance
Type pf inheritance in which the two alleles of the gene are equally dominant and so they are both expressed in the heterozygous genotype.
Alleles R red W white RR red WW white RW- pink
Calculations and codominance
The two alleles are equally dominant This kind of inheritance produces more
variation. Example
Phenotypes of parents Red X White Genotypes of parents RR X WW Gametes R and W F1- RW all pink Perform a self cross between the F1 generation
Deduce the genotypes or phenotypes of individuals in pedigree charts
Go to page 434-435.Answer exercises 2 and 4