Inheritance Flashcards
Genotype
The genetic constitution of an organism eg, all the alleles
Phenotype
The expression of the genotype and its interaction in the environment
Allele
A specific version of the same gene
Gene
Base sequence in a section of DNA that encodes for AMINO ACID SEQUENCE OF a polypeptide or functional RNA
Locus
Where a particular gene is located on a pair of homologous chromosomes
Chromosome
A molecule of DNA wrapped around histone proteins
Homologous chromosomes
Contain the same genes at the same loci, but may have different alleles
Are homologous chromosomes the same length?
Yes because they have the same genes at the same loci
But sex chromosomes are different length: only homologous at a specific sequences of bases that’s the same
Pair of Alleles at the same loci in a homologous pair do what?
Encode for a characteristic (based on amino acid sequence of a polypeptide chain) depending on the nature of the 2 alleles
Because they are the same gene encode for the same characteristic
What can each allele in a homologous pair be?
Dominant
Recessive
Co dominant
Dominant alleles
This allele will be expressed in the phenotype regardless of the nature of the other allele
Eg, will encode for the specific amino acid sequence in the polypeptide always
Recessive alleles
An allele only expressed in the phenotype if the other allele (at the same locus in a homologous pair) is also recessive
Co dominant alleles
When the 2different alleles in a homologous pair are both expressed in the phenotype
Homozygous
When the 2alleleles in a homologous pair are the SAME eg both recessive or both dominant
Heterozygous
When the 2 alleles in a homologous pair are different eg one is recessive and one is dominant
What is a mono hybrid cross used for?
To breed together 2 organisms to study the inheritance of 1 particular gene
In Gametes, how many alleles of a gene is there?
1 allele because in a gamete there are no homologous chromosomes, just 1 chromosome
So this chromosome will only have 1 allele for a gene
Punnet squares
When 2 organisms are bred together, write both their genotypes in a cross to work out the possible genotypes offspring’s can have (and ratio of what is most likely to inherit)
How could you prove an organism has a given genotype?
By using a test cross
breeding with another organism (recessive one we know the genotype for) and if an offspring with a specific genotype is produced then we can work it out
Why are the offspring produced’s genotypes not follow the ratio we calculated using punnet squares?
Because of random fusion of gametes, so the actual ratio of gametes is affected heavily by chance
Punnet square of a mono hybrid cross
2 by 2
Studying 1 gene, only 2 alleles of each organism’s homologous chromosomes can be studied
It’s only possible to have 2 alleles of 1 gene in a homologous pair
What is a di hybrid cross?
Breeding together 2 organisms to study the inheritance of alleles from 2 DIFFERENT genes
Gametes in a dihybrid cross
Every gamete will have 1 allele from 1 of the genes, then 1 allele for the other gene
Because gametes don’t have homologous chromosomes so cannot have more than 1 allele of each gene
How many possible gametes from each organism in a di hybrid cross ?
4 per organism
Each having 1 allele from 1 gene, and 1 allele from a DIFFERENT gene
How many possible gametes in a mono hybrid cross?
2 per organism because 1 gene is studied, so it’s 1 allele in 1 gamete and the other allele of the SAME GENE in the other gamete
Independent segregation of chromosomes
It is random what way the homologous chromosomes separate in meiosis, the chromosomes do not influence how each other divide
The combination of maternal and paternal chromosomes in a gamete is completely random
Dihybrid cross punnet square
4 gametes possible from mother on one axis
Same for the other
=16 possible combinations of alleles for the 2GENES
Possible genotypes in a dihybrid cross mist be written…
So alleles of the same gene are written together
Capital (dominant) allele first then recessive lowercase one
Cross
Breed together
How are co dominant alleles written?
With a capital letter to represent the same gene
Then an uppercase letter superscript to show the different allleles
ALL capital as both alleles are expressed in the phenotypen
Can genes have more than 2 alleles?
Yes but in a pair of homologous chromosomes, you can only have maximum of 2 different alleles
How are multiple alleles of the same gene written?
Capital letter to represent the gene
Superscript to show the different alleles
Sex linkage
Genes located on a sex chromosome eg X or Y
Sex chromosomes
X and Y
Non sex chromosomes
Autosomes
Carrier
For diseases caused by recessive alleles: someone is heterozygous for this allele so the disease is not expressed in the phenotype but they carry the allele for ir
For sex linked chromosome, can a father with the allele pass it on to his son?
No because the allele is on the X chromosome and a father gives the son the Y chromosome
For sex linked recessive allele on the X chromosome, why is it more likely for men to have the allele?
Because they only require 1 allele to express this in th phenotype as they only have 1 X chromosome whereas women require 2 recessive alleles on both X chromosomes
Autosomal linkage
When 2 or more genes are located on the same non sex chromosome (autosome)
Does independent segregation affect autosomal linkage?
No because the genes are on the same chromosome so when the homologous chromosomes separate, they will still be together on the same chromosome
Does crossing over affect autosomal linkage?
Yes
Crossing over
When different alleles on a homologous pair (eg within non sister chromatids) switch to form recombinant chromosomes with a new combo of alleles
How to represent autosmally linked chromosomes
Write the alleles on top of each other (in a punnet square, in gametes etc)
Effect of crossing over of autosomally linked chromosomes in meiosis?
Form RARE recombinant chromosomes aka unique combos of alleles in a chromosome that means the punnet square is extended to show the new possible combos of alleles
Epistaxis
The study of how genes interact with each other
Why are there differences in observed and expected ratio of phenotypes?
Small sample size
Random fertilisation so combination of alleles in a homologous pair is affected by chance
Crossing over occurs (rare)
Unexpected interaction between alleles (epistasis)
Crossing over mechanism
Before meiosis 1, homologous chromosomes pair up = bivalent
Form a chiasmata
And equal lengths of chromosomes between non sister chromatids are exchanged
= recombinant chromosomes with unique combo of alleles
Pure breeding meaning
Organism is homozygous for alleles of a gene
In both homologous chromosomes, has same allele for that gene
Calculating percentage of offspring with specific phenotype
Number of organisms with this phenotype worked out from cross/ number of organisms with a phenotype THAT SURVIVE PAST BIRTH
Not always every single genotype will survive so not included in possible phenotypes
Why do we know if allele is recessive or dominant
When crossed:
If phenotype is expressed in first generation the alleles for this phenotype are probably dominant
And if the allele frequency is much higher its dominant likely because only 1 copy needed for expression
