Genetics and Inheritance Flashcards
Punnet Squares and Pedigrees
what did george mendel propose
Mendel proposed the Law of Segregation after observing that pea plants with two different traits produces offspring that all expressed the same trait when the following generation expressed two traits in a 2:1 ratio.
what is the assumption of the law of segregation
- There are alternative forms for genes (known as alleles)
- For each characteristic, or trait, organisms inherit two alternative forms of that gene – one from each parent.
what is the law of segregation
a diploid individual possesses a pair of alleles for any particular trait and each parent passes one of these randomly to its offspring.
what did mendel summarise
Because all plants in F1 were tall, Mendel summarised that tallness is a dominant characteristic. He called the short characteristic recessive.
It appeared that tallness hid or masked shortness in the F1 but it did not disappear. It re-emerged in the F2.
what are the sex chromosomes of mammals
male: XY
female: XX
allele
an alternative form of a gene
genotype
the allele/variant a person carries in a particular gene. It is the genetic code or trait
eg. BB, Bb, bb
phenotype
the expression of the genotype
eg. Short or tall, black hair or white hair
homozygous
inheritance of similar alleles
eg. BB or bb
hemizygous
when you only have one allele carried on the X chromosome
This only occurs in males where they have only one X chromosome and one Y chromosome. Males can only carry one copy of the allele but they will express the allele since they don’t have another X chromosome. Unlike females, males cannot be homozygous for the condition since they don’t have two copies, so they are known as hemizygous, having half the number of X chromosome.
co-dominant
when neither allele is dominant over the other, so they are both expressed
eg. AB blood group
incomplete dominance
when neither allele is expressed, so a new trait in between is expressed
eg. Curly hair
X-linked inheritance
Genes for these traits appear on the X chromosomes, but are absent from Y chromosomes.
As there is no allelic counterpart for males with sex-linked traits, the term hemizygous is used instead of homo.
X-linked traits are mostly recessive and found more in males than in females, since the males only have one X chromosome.
X-linked, recessive (Haemophilia, red/green colour blindness) pattern of inheritance
- more males than females have the trait
- all daughters of afflicted fathers are carriers
- afflicted sons have carrier mums
X-linked, dominant pattern of inheritance
- more females than males have the trait
- all daughters of affected males have the trait
- an affected male must inherit trait from his mother
