GENETICS

Topics: Heritable Traits in ManThe Pedigree ChartApplication of Pedigree Analysis

 

Handedness - is due to a single gene with right handedness dominant and left handedness recessive. However, other scientists have reported that the interaction of two genes is responsible for this trait.

Naturally Curly Hair -early curly hair was dominant and straight hair was recessive.

Allergies - allergic reactions are induced by things a person comes in contact with, such as dust, particular foods, and pollen, the tendency to have allergies is inherited. If a parent has allergies, there is a one in four (25%) chance that their child will also have allergy problems. This risk increases if both parents have allergies.

Colorblindness - due to a recessive allele located on the X chromosome. Women have two X chromosomes, one of which usually carries the allele for normal color vision. Therefore, few women are colorblind. Men only have one X chromosome, so if they carry the allele for colorblindness, they will exhibit this trait. Thus, colorblindness is seen more frequently in men than in women.

Hairline shape - is reportedly due to a single gene with a widow’s peak dominant and a straight hairline recessive.

Widow’s Peak Hairline

Straight Hairline

Tongue Rolling – a dominant allele, R, gives some people the ability to roll their tongues into a “U” shape when it is extended. People with the recessive alleles, rr, cannot roll their tongues.

Pedigree

•The term 'pedigree' refers to the record of an organism's ancestry. •Is a pictorial representation of a family history, essentially a family tree that outlines the inheritance of one or more characteristics.•Pedigree charts are diagrams which are constructed to show biological relationships - how a trait can pass from generation to generation. •From a pedigree, you can determine if a trait is dominant or recessive, if it is sex-linked, and probability of it affecting future generations. •To build such a chart to see the flow of genotypes, symbols and rules must be followed.

Mitochondrial Genes

•Mitochondria are only inherited from the mother. •If a female has a mitochondrial trait, all of her offspring inherit it.•If a male has a mitochondrial trait, none of his offspring inherit it.

Genetic conditions caused by a mutation in a single gene follow predictable patterns of inheritance within families. Single gene inheritance is also referred to as Mendelian inheritance as they follow transmission patterns he observed in his research on peas. There are four types of Mendelian inheritance patterns:

Single Gene or Mendelian

•Autosomal: the gene responsible for the phenotype is located on one of the 22 pairs of autosomes (non-sex determining chromosomes). •X-linked: the gene that encodes for the trait is located on the X chromosome. •Dominant: conditions that are manifest in heterozygotes (individuals with just one copy of the mutant allele).•Recessive: conditions are only manifest in individuals who have two copies of the mutant allele (are homozygous).

Autosomal Dominant          

Dominant conditions are expressed in individuals who have just one copy of the mutant allele. Affected males and females have an equal probability of passing on the trait to offspring. Affected individual's have one normal copy of the gene and one mutant copy of the gene, thus each offspring has a 50% chance on inheriting the mutant allele. As shown in this pedigree, approximately half of the children of affected parents inherit the condition and half do not.

Autosomal Dominant Conditions: •  Huntington Disease •  acondroplasia (short-limbed dwarfism) •  polycystic kidney disease

The pedigree illustrates the transmission of an autosomal dominant trait

Autosomal Recessive        

   Recessive conditions are clinically manifest only when an individual has two copies of the mutant allele. When just one copy of the mutant allele is present, an individual is a carrier of the mutation, but does not develop the condition. Females and males are affected equally by traits transmitted by autosomal recessive inheritance. When two carriers mate, each child has a 25% chance of being homozygous wild-type (unaffected); a 25% chance of being homozygous mutant (affected); or a 50% chance of being heterozygous (unaffected carrier).

Affected individuals are indicated by solid black symbols and unaffected carriers are indicated by the half black symbols.Autosomal recessive diseases: •  Cystic fibrosis •  Tay-Sachs •  hemochromatosis •  phenylketonuria (PKU)

X-linked Recessive          

X-linked recessive traits are not clinically manifest when there is a normal copy of the gene. All X-linked recessive traits are fully evident in males because they only have one copy of the X chromosome, thus do not have a normal copy of the gene to compensate for the mutant copy. For that same reason, women are rarely affected by X-linked recessive diseases, however they are affected when they have two copies of the mutant allele. Because the gene is on the X chromosome there is no father to son transmission, but there is father to daughter and mother to daughter and son transmission. If a man is affected with an X-linked recessive condition, all his daughter will inherit one copy of the mutant allele from him.

X-linked Recessive Disorders: •  Duchenne muscular dystrophy •  hemophilia A •  X-linked severe combined immune disorder (SCID) •  some forms of congenital deafness

X-linked Dominant          

Because the gene is located on the X chromosome, there is no transmission from father to son, but there can be transmission from father to daughter (all daughters of an affected male will be affected since the father has only one X chromosome to transmit). Children of an affected woman have a 50% chance of inheriting the X chromosome with the mutant allele. X-linked dominant disorders are clinically manifest when only one copy of the mutant allele is present.

X-linked Dominant Disorders •  some forms of retinitis pigmentosa •  Chondrodysplasia Punctata •  hypophosphatemic rickets

Application of Pedigree Analysis

Huntington disease

Albinism

Cystic fibrosis

The roman numerals indicate the generation (I, II or III) and the numbers show birth order of children (1, 2, 3, 4 etc.)

a. Assuming this is a recessive gene, we know that II-3 and III-4 must be “aa.”

b. Therefore both parents of these offspring must have given an “a” allele.

c. So, I-1, I-2, II-4 and II-5 must each have one “a” in their genotypes.

d. Since I-1, I-2, II-4 and II-5 don’t have the disease, their other allele must be “A.”

e. All the rest of the people could be either AA or Aa.