heredity

What is a gene?

It is a portion (part) of the chromosome (DNA) which is responsible for a certain

hereditary trait.

 Notion of the gene:

Gene protein synthesis Protein phenotype (trait)

 A diploid cell has homologous chromosomes which have same genes on same locus.

 What is an allele?

It is a copy or version of a gene.

Ex : Gene for the color of eyes has several alleles , Black , brown , blue .....

- Dominant allele is expressed and always masks the recessive allele.

- Recessive allele is not expressed and is masked (hidden) by the dominant allele.

N.B. Use the first letter of the trait as a symbol of the allele

Dominant allele is symbolized by upper case ex. Brown ...B

Recessive allele is symbolized by lower case ex. White ...w

 What is a genotype?

Genotype is the set of alleles of an individual. Example BB, gg, Bg

- Organism having two identical alleles of a gene is said to be homozygous or pure for

that gene. Ex. AA, BB, OO

- Organism having two different alleles of a gene is said to be heterozygous or hybrid

for that gene. Ex. AB, AO, BO

 What is a phenotype?

It is the set of observable characteristics of an individual. A phenotype results from the

expression of a genotype.

NATIONAL EVANGELICAL INSTITUTE FOR GIRLS AND BOYS

مدرسة الفنون اإلنجيلية الوطنية للبنات والبنين

(Sequence of

DNA

nucleotides)

(Sequence of

amino acids)

2

 Example about Blood types

- The gene for blood types is located on chromosome 9, it has 3 alleles A, B, and O.

- Alleles A and B are dominant while allele O is recessive.

Blood type (phenotype) Genotype

Blood type[A] AA homozygous (pure)

Or

AO heterozygous (hybrid)

Allele A is dominant, allele O is

recessive.

Allele A is expressed while

allele O is masked by allele A

Blood type [B] BB homozygous (pure)

Or

BO heterozygous (hybrid)

Allele B is dominant, allele O is

recessive.

Allele B is expressed while allele

O is masked by allele B

Blood type [O] OO homozygous (pure) Allele O is recessive and is

always expressed as pure.

Blood type [AB] AB heterozygous (hybrid) Alleles A and B are codominant.

Both alleles are expressed.

- A dominant phenotype may be expressed as a pure genotype or as a hybrid genotype.

- A recessive phenotype is always expressed as pure genotype.