Inheritance - M6 Flashcards
Gene
A length of DNA that codes for the production of a polypeptide
Allele
A variant form of a particular gene
Locus
The position of a gene on a chromosome
Dominant
Represented with an upper case letter.
Has the same effect on the phenotype whether one or two alleles are present
Recessive
Represented with a lower case letter.
Only affects the phenotype if its homozygous
Codominant
Both alleles of one gene are dominant
Both alleles affect the phenotype in a heterozygous individual
Eg: white and red dominate allele = pink phenotype
As a result, both alleles are expressed in the phenotype of the organism if present.
Heterozygous
A genotype where the two alleles are different. E. G: Aa
Homozygous
A genotype where the two alleles are the same. E.g: AA or aa
Heterogametic
Possession of two different sex chromosomes. E.g: XY in human males
Homogametic
Possession of the same sex chromosomes.
XX in human females
Genotype
The genotype of an organism refers to the alleles of a gene possessed by that individual.
Phenotype
The expression of the alleles, giving the individuals observable characteristic
Monohybrid cross
A genetic cross involving one gene
Dihybrid cross
A genetic cross involving two GENES (on different chromosomes)
Linkage
Two genes on the same chromosome are said to be linked
Sex linkage
characteristic is sex linked if the genes that code for it are Genes with a locus on one of the sex chromosomes. In humans normally on the X
Haemophilia A
A sex linked disease that stops blood clotting.
The gene is found on the X chromosome
caused by a recessive allele of a gene that codes for a faulty version of the protein factor VIII
X^H = normal allele
X^h = haemophillia allele
Sickle cell anaemia
An example of a disease caused by codominant alleles. The red blood cells change shape and can cause clots
Epistasis
when two genes on different chromosomes affect the same feature
F1 generation
When a homozygous dominant individual is crossed with a homozygous recessive individual the offspring
All of the F1 generation are heterozygous
F2 generation
The offspring produced when two of the F1 generation are crossed
Chi-squared
A statistical test to test the significance of the difference between observed and expected results
Genome
The sum total of genes in an Organism
Expected ratio of F1 generation of heterozygous dihybrid cross (YyRr x YyRr)
9:3:3:1
Expected ratio of recessive epistasis
9:3:4
Dominant epistasis expected ratio
12:3:1
Recessive epistatic alleles
If one GENE is recessive, it will mask the other GENE
e.g: (E.g. Flower pigment is controlled by two genes. Gene 1 codes for a yellow pigment (Y) and gene 2 codes for an enzyme that turns the yellow pigment orange (R)
If a plant is homozygous recessive for the epistatic gene (yy) (yellow allele isn’t expressed) then it will be colourless, masking the colour of the orange gene (R))
Dominant epistatic alleles
When the epistatic allele is dominant, then having at least one copy will mask the expression of the other gene.
(E.g. Squash colour is controlled by two genes – the colour epistatic gene (W/w) and the yellow gene (Y/y))
If Ww or WW then squash will be colourless
if ww, Yy or YY = yellow, yy = green
Autosome
not a sex chromosome
autosomal linkage
Genes that are on the same autosome (not a sex chromosome)
genes on same autosome are said to be linked - they’ll stay together during independent assortment meiosis 1
Won’t happen if crossing over splits them up
-closer the genes are on the chromosome, the more linked they are
most X linked conditions are found in ______
males
chi squared test is used to
test if your results fit the pattern you expected
Null hypothesis for chi squared
THere is no significant difference between the observed + expected results
chi squared how to calculate degrees of freedom
The degrees of freedom can be worked out from the results. It is calculated by subtracting one from the number of classes
n-1
example of Multiple Alleles for one gene
Blood type.
I^A , I^B , I^O
where A and B are codominant and O is recessive to both
Blood type Genotype
I^A , I^B
gives phenotype :
Type AB
“Universal receivers as immune system sees both antigens as A and B (type o codes for no antigen)
Why are Males more likely to be affected by sex linked diseases
Males have XY chromosome
More easily affected by alleles on X
(X chromosome is larger than Y)
therefore no alleles on Y to “counteract” it
Why are females less likely to be affected by sex linked diseases
Females have XX chromosomes
have a different allele on other X chromosome which does not code for disease :. would counteract it
How many possible phenotypes arise from Codominance
3
e.g: white, red and pink (Codominant bc red + white)
How to represent alleles in codominance
why?
superscripted
e.g: I^W , I^R
for white and red petal colours
RW would represent two different genes (epistasis)
what leads to genetic variation in sexual reproduction
- Crossing over of non-sister chromatids during prophase I of meiosis
- Independent assortment of homologous chromosomes during metaphase I of meiosis
- Random fusion of gametes during fertilisation
what leads to Unexpected ratios in inheritance
-Random fertilisation -> can’t predict which gametes are successful
-Autosomal linkage (genes which are inherited as a package due to closeness to each other on chromosome) + no crossing over to separate these genes (crossing over to separate genes is less likely if they are closer together on chromosome because they would be swapped together if swapped )
-epistasis
To test if there is linkage or epistasis
(causing unexpected ratios), you must …
Chi squared test. - find significant difference between expected + observed ratios of offspring
When is a Chi squared calculated value Significant
If the chi squared value is GREATER than the critical value (0.05)
X^2 > 0.05
(opposite to in maths)
a particular gene that is found only on the X chromosome has two alleles G and g. The genotype of a heterozygous female would be written as …….. . A males dom genotype would be written as …….
X^GX^g
X^GY
Phenotypic variation is
the difference in phenotypes between organisms of the same species
Different environments around the globe experience very different conditions in terms of the:
5
(for plants)
-Length of sunlight hours (which may be seasonal)
-Supply of nutrients (food)
-Availability of water
-Temperature range
-Oxygen levels
Some questions in the exam may ask you to explain why the variation in phenotype due to genetics is inherited but the variation in phenotype due to environmental factors is not. This is because
genetic variation directly affects the DNA of the gametes but variation in phenotype caused by the environment does not.
The small differences in DNA base sequences between individual organisms within a species is called
genetic variation
To work out the number of different possible chromosome combinations the formula ……. is used where n =
2^n can be used, where n corresponds to the number of chromosomes in a haploid cell
For humans this is 2^23 which calculates as 8 324 608 different combinat
mechanism of independent assortment of homologous chromosomes during metaphase 1
random allignemnet of chromosomes results in random different combinations of chromosomes and different allele combinations in each gamete
= genetic variation between gametes produced by an individual
Mechanism of crossing over of non-sister chromatids during PROPHASE 1
Exchange of genetic material between non-sister chromatids leads to new combinations of alleles on chromosomes. Can also break linkage between genes
what are recombinant offspring
offspring that ahave a different combination of characteristics to their parents
what are parental type offspring
offspring that have the same combination of characteristics to their parents
f a large number of genes have a combined effect on the phenotype they are known as
polygenes
Discontinuous variation:
At the genetic level:
Different genes have ………….. effects on the phenotype
Different alleles at a single gene locus have a ……… effect on the phenotype
different
large
Continuous variation:
At the genetic level:
Different alleles at a single locus have a …….. effect on the phenotype
Different genes can have the same effect on the phenotype and these add together to have an ………. effect
small
additive
