3.7.1 Inheritance Flashcards
What’s genotype
Genetic constitution of an organism
What’s phenotype
Expression of genetic constitution (genotype) and it’s interaction with the environment
What’s a gene
Section of DNA which is made of a sequence of nucleotide bases, codes for a polypeptide
What’s an allele
A different form of a gene
There can be many alleles of a single gene
Only ONE allele can occur at the locus of a chromosome
diploid organisms have homologous chromosomes, two loci which carry two alleles
What are homozygous alleles at a locus
Alleles at both loci of each chromosome are the SAME
What are heterozygous alleles at a locus
Two alleles at the loci (locations) of each chromosome are different
What’s a dominant allele
Allele in the heterozygote which is expressed in phenotype
What’s a recessive allele
Allele in the heterozygote which isn’t expressed in phenotype
What are homozygous dominant alleles
Homozygous organism with two dominant alleles
What are homozygous recessive alleles
Homozygous organism with two recessive alleles
What’s codominant alleles
Both alleles contribute to phenotype
Phenotype is a blend of both features
Steps to produce a monohybrid cross
Cystic fibrosis causes by a recessive allele, two carriers reproduce find probability they will have a child with cystic fibrosis AND probability they have a girl with cystic fibrosis
- Define terms
B= no cystic fibrosis
b= cystic fibrosis - Parental phenotype
Carrier X Carrier
Parental genotypes
Bb X Bb
Gametes (monohybrid = 1 letter)
(B) (b) X (B) (b) - Punnet square
BB :Bb Bb :bb - Ratio and Phenotype
Not affected: carrier: sufferer
1:2:1 - 25% chance of a child with cystic fibrosis
0.5 x 0.25 = 12.5% change of a girl with cyclic fibrosis
Difference between a monohybrid cross and dihybrid cross
Monohybrid, focuses on one trait, one letter in each gamete
Dihybrid, focuses on two different traits at the same time, two letters in each gamete
Co dominance (monohybrid or dihybrid crosses) this is just colour so monohybrid
Cows are red white or roan in colour.
Red and white are dominant, if two roam cows reproduce find probability they produce red offspring
- Define terms (co dominance so parents are heterozygous)
C^R = red
C^W = white - Parental phenotypes
Roan X roan
Parental genotypes
C^R C^W X C^R C^W
Gametes
(C^R) (C^W) X (C^R) (C^W) - Punnet square
C^R C^W
C^R C^W
C^R C^R
C^W C^W - Phenotypes and ratio
Roan: white : red
2: 1: 1 - Probability
25% probability of a red cow
How is co dominance written
Gene^allele
Allele is capital if dominant, lower case is recessive
Colour of a cow can be red (dominant or white (dominant) or roan (recessive)
C^W white
C^R red
Steps to produce a dihybrid cross (2 different traits at the same time)
Dihybrid cross between a round (dominant) yellow (dominant) pea and green (recessive) wrinkled (recessive) pea
Part 2 Cross two round and yellow offspring together
Basic question try harder ones
- Define terms
R= round r= wrinkles
Y= yellow y= green - Parental phenotypes
Round and yellow X wrinkled and green
Parental genotypes
RR YY X rr yy
Gametes (two in each circle bc two traits)
(RY) X (ry) - Punnet square
Rr Yy - Phenotype and ratio
Round and yellow
All offspring are round and yellow
Part 2
- Parental phenotype
Round and yellow X round and yellow
Parental genotype
RrYy X RrYy
Gametes (two in each gamete)
(RY) (Ry) (rY) (ry) X (RY) (Ry) (rY) (ry) - Punnet square
RR YY
RR Yy
Rr Yy
Rr YY
RR Yy
Rr Yy
Rr YY
Rr Yy
Rr Yy
RR yy
Rr yy
Rr yy
rr YY
rr Yy
rr Yy
rr yy
- Phenotype and ratio
Yellow and round: yellow and wrinkled : green and wrinkled: green and wrinkled
9:3:3:1
What’s multiple alleles and how is it written
A gene with more than 2 alleles
Gene^allele
Different blood groups A B and O
B^O
B^A
B^B
How to draw a genetic cross for multiple alleles this one is w/ co dominance
Parents with blood group AB and O reproduce
I^A and I^B are co dominant
I^O is recessive
Probability of having offspring with blood group A
- Define terms
I^A = A
I^B = B
I^O = O - Parental phenotypes
Blood group AB X blood group O
Parental genotype
I^A I^B X I^O I^O
Gametes
(I^A)(I^B) X (I^O) - Punnet square
I^B I^O
I^A I^O - Phenotype and ratio
Blood group B : blood group A
1:1 - 50% chance of having offspring with blood group A
What’s sex linkage and how is it written
Females are XX
Makes are XY
A gene carried on the X or Y chromosome
However there is no equivalent homologous portion on the Y chromosome
X^ allele Y^allele
Dominant allele = capital letter
Recessive allele = lowercase
X^A X^A or X^a Y
Steps for a sex linked cross (if Q is sex linked it will state it in question)
Colour blindness caused by recessive allele on X chromosome (stated it’s sex linked)
Non colour blind male reproduces with female carrier find probability child is colour blind
- Define terms
X^R= not affected
X^r= colour blind - Parental phenotypes
Non colour blind X carrier
Parental genotypes
X^R Y X X^R X^r
Gametes
(X^R) (Y) X (X^R) (X^r) - Punnet square
X^R X^r
X^R X^R
X^r Y
X^R Y - Phenotype and ratio
Normal girl: normal boy: colour blind girl: colourblind boy
2:1:0:1 - Probability
25% chance if a child being colour blind
Why are sex linked conditions/chromosomes more common in males
Males have XY only one copy of a recessive allele needs to be inherited to have the condition (50%chance)
Whereas women must inherit 2 recessive alleles
How to know if a condition is sex linked from a family tree
Look for fathers and daughters
If a daughter has the condition the father must have it too as she will inherit X^recessive from mum and X^recessive from dad
How to prove it a condition is recessive from a family tree
Look for unaffected parents with an affected child
If child is a boy and condition is sex linked one parent must carry a recessive allele
Otherwise parents must be heterozygous (both parents carry a recessive allele)
What’s epistasis
One allele masks or affects the expression of another allele
Eg B=melanin
b=no pigment
A= black stripes
a= full black coat
If offspring is bb AA
It will be white/not coloured as black stripes can’t be expressed if there isn’t any pigment
Why are the ratios from a cross not always correct
Each fertilisation is a separate random event
Tips
Know dihybrid and monohybrid crosses
Ensure you know how to write co dominant and sex linked alleles
Memorise facts ie proving sex linkage and why ratios aren’t always correct
Practise questions
What’s crossing over in meiosis and when does it occur
Produce new combinations of alleles in gametes
Both alleles are present on the same chromosome, they swap sections with a non sister chromatid
Crossing over occurs infrequently
If the genes are close together, even less likely to cross over
What’s an autosome and autosomal linkage
Autosome, Chromosomes that aren’t sex linked
Autosomal linkage, 2 or more genes are carried on the same autosome (chromosome)
Alleles on each chromosome are linked and therefore inherited together
Autosomal linkage with no crossing over
Parent 1 is RrYy
Parent 2 is RrYy
What are the offspring genotypes
Parent 1
R| |r
Y | |y
Parent 2
R| |r
Y | |y
Gametes
(RY) (ry) X (RY) (ry)
Punnet square
Offspring and ratio
RRYY :RrYy :rryy
1:2:1
Autosomal linkage with crossing over
Parent 1RrYy
Parent 2 RrYy
Parent 1
R| |r
Y | |y
Parent 2
R| |r
Y | |y
Gametes
The Y and y can cross over between two chromatids, 4 possible options of gametes for each parent
(RY) (Ry) (rY) (ry) X (RY) (Ry) (rY) (ry)
Punnet square
