Autosomal inheritance:-
Transmission of characters other than the sex linked traits from parents to their offspring is autosomal inheritance.
Autosomes control variety of characters other than sex.
Such characters are called as autosome linked traits.
Autosomal inheritance can be either dominant type or recessive type.

A) widow’s peak:-
1) It is determined by autosomal dominant gene.
2) A prominent ‘V’ shaped hairline is seen on forehead.
3) It occurs in homozygous dominant condition (WW) or heterozygous condition (Ww).
4) Homozygous recessive (ww) will never have widow’s peak.
5) Shows equal chances of expression in males and females.

 

B) Phenylketonuria:- (autosomal recessive trait)

A defective gene (genetic mutation) causes PKU, which can be mild, moderate or severe.
In a person with PKU, this defective gene causes a lack of or deficiency of the enzyme that's needed to process phenylalanine, an amino acid.
A dangerous buildup of phenylalanine can develop when a person with PKU eats protein-rich foods, such as milk, cheese, nuts or meat, and even grains such as bread and pasta, or eats aspartame, an artificial sweetener. This buildup of phenylalanine results in damage to nerve cells in the brain.

PKU signs and symptoms can be mild or severe and may include:

•      A musty odor in the breath, skin or urine, caused by too much phenylalanine in the body

•      Neurological problems that may include seizures

•      Skin rashes (eczema)

•      Fair skin and blue eyes, because phenylalanine can't transform into melanin — the pigment responsible for hair and skin tone

•      Abnormally small head (microcephaly)  Hyperactivity

•      Intellectual disability    Delayed development

•      Behavioral, emotional and social problems

•      Psychiatric disorders

Q.: What is sex linked genes? Explain its types in detail?

Ans.: 1) The genes located on non-homologous regions of sex chromosomes but involved in sex determination are called as sex linked genes.

2) The body characters controlled by sex linked genes are known as sex linked characters.

3)  The phenomenon of inheritance of sex - linked genes is called as sex linked inheritance or sex linkage.

4)  There are two types of sex linked genes.

A) X- linked genes / sex linked genes:

1)   Genes located on non-homologous part of X - chromosome only are called as X - linked genes.

2)  These genes show no corresponding allele on Y - chromosome.

3)  The recessive sex linked genes in humans are responsible for sex linked characters like hemophilia, colour blindness, night blindness, myopia, muscular dystrophy etc.

4)  Females shows two X-chromosomes and a single X-chromosomes is present in males.

Due to this reason two recessive sex linked genes are necessary for expression of recessive characters in female whereas a single recessive   gene is sufficient to express   recessive character in males.

5)  If a female receives recessive gene over one X- chromosomes the dominant gene if present on other X- chromosomes will suppress its expression.

     Such a female will never express recessive character.

     6) The female with one recessive gene is called as carrier.

     7) Very rarely any female individual will show both recessive genes on X- chromosomes.

8)  Males suffer more from phenotypic expression of recessive sex linked gene as they don’t have homologous X-chromosomes or allelic pair on Y-chromosome.

9)  Show criss - cross inheritance.

B)  Y - linked genes / holandric genes:

1)   The genes located over non homologous part of Y - chromosomes only are called as Y - linked genes.

2)  They inherit along Y - chromosomes and are expressed only in males.

3)  Excessive growth of hair on external ear or pinna i.e. Hypertrichoisis is an example.

Give reasons: Sex linked characters appear more frequently in men than in women.

Ans.: 1) Human male shows a pair of heteromorphic sex chromosome i.e. XY while a female shows homomorphic pair as XX.

2)  Since, Y - chromosome is present in males only and not in females
Y-linked genes will be expressed only in males.

3)  The sex linked dominant genes present on X-chromosome are expressed equally in both males and females. But sex linked recessive gene are expressed only when both chromosome show recessive genes.

4)  Since male contain only one X- chromosome he requires only a single recessive gene for expression of sex linked gene.

Due to all above reasons, sex linked characters appear more frequently in men than in women.

Q.: Write a note on colour blindness?

     Ans. -

1)   The sex linked disease in which a person is unable to distinguish between red and green colour as both these appears him or her grey is known as red greed colour blindness.

2)  The disease develops due to recessive X - linked genes over X - chromosome.

3)  It prevents formation of colour sensitive cells in the retina necessary for distinction of red and green.

4)  The genes for normal vision (dominant) and colour blindness (recessive) are located on non-homologous region of X - chromosome and absent on Y - chromosomes.

5) If the gene for normal vision is represented by XC and gene for colour blindness is Xc then the genotypes of different individuals can be represented as.....

 

     Sex       Normal       Colorblind       Carrier

      Male           XCY                  XcY                      -

      Female        XCXC                        XcXc                XCXc

6)  The disease show criss cross inheritance and can be explained as follows:

A)  If a colorblind man marries a woman with normal vision then all the offspring will have normal vision.

      Parent:    Colorblind male              X      Normal Female

     Genotype           XcY                    X             XCXC

 

 

      Gamete        Xc        Y                         XC           

 

      Checker Board:

                                Xc                            Y

         XC                   XCXC                       XCY

                        Carrier female           Normal Male

         XC                   XCXc                        XCY

                        Carrier Female          Normal Male

 

Result:   1) All offsprings with normal vision.

                  2) All daughters with normal vision but carrier for disease.

                  3) All sons with normal vision.

B) If a carrier female (XCXc) marries a male with normal vision (XCY) then,

Parent:          Normal Male         X      Carrier Female

Genotype            XCY          X      XCXc

 

Gametes         Xx      Y                      XC   Xc

Checker Board :

 

                        XC                            Y

                        XCXC                                XCY

         XC           Normal female                Normal Male

         Xc           XCXc                                XcY

                     Carrier Female              Colorblind Male

 

      In the above marriage four genotypically different offsprings will be obtained.

Result:

1) 75 % offsprings will have normal vision and 25 % will be colorblind.

2) Among sons, 50 % will be normal and 50 % will be colorblind.

3)  Among daughters 100% will be normal vision but 50% daughters will be carriers for recessive gene.

4)  From above two example, it is clear that in colorblind father transmits his gene to his grandson via daughter which is carrier.

The inheritance of characters from father to his grandson through his daughter is called as crisscross inheritance or digenic type of inheritance.

Q.: Explain hemophilia with suitable example.

Ans.: 1) Hemophilia is hereditary blood disease in which blood clots slowly or fails to clot.

2) The person which carries the recessive gene for hemophilia has deficiency of clotting factors VIII or IV in blood. So, minor injuries cause continuous bleeding. Hence the disease is also called as bleeder’s disease.

3)  The genes for normal clotting (dominant) and the hemophilia (Recessive) are located on non-homologous region of X- chromosome but their alleles are absent on Y - chromosome.

4)  If the gene for normal clotting is XH and gene for hemophilia is Xh then the different genotypes of different individuals can be denoted as follows -

     Sex       Normal            Hemophilic        Carrier

Male           XHY                XhY                  -

Female       XhXh              XhXh                        XHXh

5)  The inheritance can be studied as follows -

A)  If a hemophilic male (x h y) marries a females with normal clotting of blood (XHXH) then all the offsprings will have normal clotting of blood.

Parent       Hemophilic      X      Normal

                  Male                       Female

Genotype     Xhy         X      XH XH

 

Gametes     Xh   Y                      XH   XH

 

     Fi. Generation:

                     Xh                         Y

                     XHXh                   XHY

     XH          Carrier female      Normal Male

                      XH Xh                     XHY

     XH          Carrier female      Normal Male

Result: 1) All offspring will have normal blood clotting.

            2)   All sons will have normal blood clotting.

            3)   All daughters will be carrier with normal blood clotting.

B)  If a carrier female (XHXh) marries a male with normal clotting (XHY), then,

     Parent:        Normal Male        X      Carrier Female

      Genotype :   XH Y                   X      XH Xh

 

      Gametes:    XH Y                                 XH Xh

    Checker Board:

                     XHXH                          XHY

            XH    Normal Female              Normal Male

                     XHXh                           XhY

            Xh     Carrier Female              Hemophilic Male

 

    Result:

     1) Four different types of genotypes will be obtained.

     2) 75 % offsprings will show normal blood clotting and 25 % will be hemophilic.

     3) Among sons 50 % will be normal and 50 % will be hemophilic.

     4) Among daughters 50 % will be normal and 50 % will be carrier with normal clotting.

    5)  Similar to colorblindness, hemophilia also follows criss cross inheritance or digenic inheritance.

REFER TABLES ON PAGE NO. 63 AND 64 STATE BOARD BOOK