Topic 1 Notes

Topic 1 Notes

Gregor Mendel
Father of modern genetics; combined plant breeding, statistics, careful record-keeping; described hypothesis of transmission of traits now considered the laws of inheritance. Mendel studied pea traits with two distinct forms:

For example, Mendel studied pea plant height:
• Mendel observed that crossing tall x tall yields tall plants.
• In other cases, he observed that tall x tall yields both tall and short plants.
• Mendel concluded that some plants were "pure" and others were "hybrids."
• Hybrid plants carry the recessive short trait (which is hidden, but not absent) and show the dominant tall trait.
• The observed trait is dominant; the masked trait is recessive.

Phenotype Genotype Description
Tall plant TT Homozygous dominant ("tall"-associated alleles)
Tall plant Tt Heterozygous (sometimes called a carrier)
Short plant tt Homozygous recessive ("short"-associated alleles)

Alleles = Versions of the same gene or DNA sequence; the DNA sequence differs at one or more sites.
• Homozygous: Cell contains two alleles that are the same (TT or tt).
• Heterozygous: Cell contains two alleles differ (Tt).
• Dominant: An allele that produces an effect even when present in just one copy; represented by a capital letter.
• Recessive: An allele that needs to be present on both chromosomes (two copies) in order to be expressed; represented by a lower case letter.

Phenotype = The observed trait or physical appearance.

Genotype = The combination of alleles present; the underlying instructions.





Law of segregation = States that parental alleles separate into different gametes during meiosis, such that each gamete contains only one allele for each trait. Parental cells have two alleles; gametes have one allele.

Punnett square
Used to predict the possible offspring that result from two parents. These predictions can be represented as ratios. It is important to note that the observed (real-life) ratios may not match the calculated ratios when looking at small numbers of offspring.

Monohybrid cross (Tt x Tt)

Two heterozygous tall parents (Tt) will produce gametes that contain either:
T (dominant, tall allele)
t (recessive, short allele)

A Punnett Square is used to predict potential offspring arising from this cross (Tt x Tt):
Offspring ratios
Genotypes: 1/4 TT : 2/4 Tt : 1/4 tt
Phenotypes: 3/4 tall : 1/4 short
The recessive short allele (t) is "hidden" in the heterozygote (Tt) offspring.

Modes of inheritance = Describes the patterns through which a particular phenotype (such as a disorder) is inherited.
• Autosomal dominant inheritance
• Autosomal recessive inheritance
• X-linked dominant inheritance
• X-linked recessive inheritance
• Y-linked inheritance


Autosomes are the "non-sex" chromosomes found in both genders.

Sex chromosomes (X and Y) are gender-specific. "X-linked" inheritance or "Y-linked" inheritance simply means that a gene is located on the X or Y chromosome, respectively. Sex-linked inheritance will be discussed more later on...

Autosomal dominant inheritance
For the inheritance of a disorder that is DOMINANT:
• Heterozygotes (Aa) inherit the dominant allele and exhibit the "affected" phenotype. Since the disorder is dominant, only one copy is needed; the recessive "unaffected" allele (a) is masked by the dominant "affected" allele (A).
• Males and females are equally affected since it is not associated with a sex chromosome' males and females may equally transmit the trait.
• The affected phenotype does not skip a generation. If the dominant allele is passed on, that offspring will express the affected phenotype.

Example: Parents: Aa x aa
Only Aa is affected by the dominant disorder (A), since aa has two recessive/normal alleles.
Genotypic ratios of potential children:
2/4 Aa : 2/4 aa
Phenotypic ratios of potential children:
2/4 affected (Aa) : 2/4 unaffected (aa)
50% affected : 50% unaffected

Autosomal recessive inheritance
For the inheritance of a disorder that is RECESSIVE:
• Heterozygotes (Cc) carry the recessive (affected) allele but exhibit the dominant (unaffected) phenotype. Since the disorder is recessive, two recessive alleles (cc) are needed in order to observe the disorder.
• Males and females are equally affected and may transmit the trait.
• The expression of the recessive "affected" trait may appear to skip generations if it is masked by a dominant "unaffected" allele.

Example: Parents: Cc x Cc (carriers)
Both parents are unaffected since they each have one dominant normal (C) allele. They are carriers of a recessive disorder.
Genotypic ratios of offspring:
1/4 CC : 2/4 Cc : 1/4 cc
Phenotypic ratios of offspring:
1/4 unaffected : 2/4 carriers : 1/4 affected
75% unaffected : 25% affected