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What can allele be
Genes have different forms called alleles. An allele can be recessive or dominant.
What happens when a dominant gene is paired with a recessive gene
The dominant allele determines the characteristic
How can a recessive gene produce a recessive phenotype
the individual must have two copies, one from each parent
What is a genotype
The genotype is the set of alleles of a gene in our DNA responsible for a particular trait.
What is a phenotype
The phenotype is the physical expression, or characteristics, of that trait.
Genotype and phenotype relationship
A phenotype is how the genotype is expressed.
Homozygous
when both alleles are the same
Heterozygous
when alleles are different
AA
Homozygous dominant
Aa
Heterozygous
aa
homozygous recessive
Hemizygous
only one copy of gene instead of two. All the genes on the single X chromosome in males are hemizygous
Monohybrid cross
Mating between two individuals, investigating one gene
Male and female sex chromsomes
Male; XY
Female; XX
However, there are variations XXY (Klinefelter syndrome) or XO (Turner syndrome).
Sex linked
Why do X-linked recessive conditions occur more often in males?
Recessive trait cannot be hidden since there is no second allele
Whereas females with their second X get a second chance.
ONLY females can be carriers.
Sex linked conditions dominant/recessive
It must be carried on either an X or Y chromosome.
Typically, there are more X linked conditions
since the x chromosome it much bigger than the Y chromosome and therefore can carry more genes.
What are the mendal laws
Law of dominance
Law of segregation
Law of Independent assortment
What is the law of dominance?
In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.
What is the Law of segregation?
During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
What is the Law of Independent assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another.
What traits don't follow the dominant/recessive rules of Mendels
Codominance
Incomplete dominance
Multiple Alleles
Lethal alleles
Multiple Genes (polygenic)
Environmental Factors (epigenetic)
What is incomplete dominance
Ratio will not fit the expected dominant / recessive pattern
What happens in incomplete dominance?
A heterozygous genotype will produce a mixed phenotype.
This new, third phenotype is a blend of both parents, cannot clearly see either parental phenotype.
Since neither phenotype completely hides the other, there is no dominant or recessive allele.
Codominance meaning
Heterozygote produces a third phenotype. Both parental phenotypes are clearly seen.
Multiple Alleles meaning
It is possible that a trait has more than two (multiple) alleles that determine which phenotype is expressed.
Ex.
hair colour in rabbits
blood groups in
humans.
Multiple alleles and Codominance in ABO Blood Group
The ABO blood group system is the classification of human blood based on the inherited properties of red blood cells depending on the antigens A and B, which are carried on the surface of the red cells.
People may have type A, type B, type O, or type AB blood.
