Short notes on Sex Determination, Mutations and Genetic Disorders For CBSE & NEET

Our genes determine everything about our development. So its important to learn about how genes are inherited and what would happen if there is any error in the inheritance of genes. This article discusses the inheritance patterns that determine the gender of an organism, and also discusses briefly about mutations and the disorders due to them.

Sex determination

(a) XX Female and XY Male Types:

The XY sex determination system is the determination of sex via sex chromosomes in most mammals (including humans), some insects and some plants. In this system, males are the heterogametic sex, XY, while females are the homogametic sex, XX.

In fertilisation, an ovum can only carry an X chromosome. It is therefore the sex chromosome found in the sperm – either X or Y – that determines whether a zygote will be female or male, respectively.

(b) ZW Female and ZZ Male Type:

In butterflies and birds, the female is heterogametic having dissimilar Z and W chromosomes whereas the male is homogametic having similar ZZ chromosomes (It is a convention to designate female as ZW instead of XY and male as ZZ instead of XX). The situation here is just reverse to first type.

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(c) XX Female and XO Male Type:

Mc Clung and Wilson (1903) described this type of sex mechanism in insects especially in grasshopper. In male there is no mate for X chromosome, hence the name XO is given, there is no Y chromosome. They produce sperm of two types, 50% with X chromosome and 50% without X. In females there are two similar or homomorphic sex chromosomes XX.

Genic Balance Theory:

This theory was proposed by Calvin Bridges (1921). From his study on Drosophila he suggested that the X chromosomes carry factor for femaleness whereas autosomes “A” carry genes for maleness. Y chromosome does not take part in sex determination. 

The genic balance is governed by the ratio of the number of X-chromosomes to the number of sets of autosomes in the zygote at fertilization.

If the ratio is 1.0 the offspring develops into female, but if it is 0.5, then the offspring develops into male. If the ratio is intermediate between 1.0 and 0.5, the resulting individual is neither a male or nor a female but an inter sex.  Super females have a ratio of 1.5 and super males have a ratio 0.33.

Mutations:

It is a phenomenon, which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism. This phenomenon also leads to variation in DNA.

1. Aneuploidy : 

The failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosomes called aneuploidy.

It is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. A cell with the correct number of chromosomes is called a euploid cell.

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2. Polyploidy: The failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organisms. Complete extra sets (3n, etc.) – fatal in humans, most animals.

3. Nondisjunction – mistake in cell division where chromosomes do not separate properly in anaphase, usually in meiosis, although in mitosis occasionally; in meiosis, can occur in anaphase I or II.

Genetic Disorders

Genetic disorders can be divided into two main categories:

Mendelian Disorders

These disorders occur mainly due to the alteration or mutation in a single gene. These disorders may be dominant or recessive. These are transmitted from one generation to the next following Mendel’s principles of heredity.

Examples of Mendelian disorders are haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria, thalassemia, etc.

1. Haemophilia
Haemophilia (Bleeder’s disease) is X-linked recessive disease, which was first studied by John Cotto in 1803.
Haemophilia was common in royal families of Europe. Haemophilia-A is characterised by lack of anti-hemophilic globulin (factor-VIII).
Haemophilia-B or Christmas disease results from a defect in Plasma Thromboplastic Component (PTC or factor-IX).

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2. Colour Blindness

It is a condition, in which certain colours cannot be distinguished, due to the lack of one or more colour absorbing pigments in the cone cells of retina.
In humans, the most common colour blindness is red-green colour blindness, which is a sex linked (i.e. X-linked recessive) defect caused by a recessive gene and is thus more common in males than females.

3. Sickle-cell Anaemia
This is an autosome-linked recessive trait that can be transmitted from parents to the offspring when, both the partners are carrier or affected for the gene.

The disease is carried by a single pair of allele, HbA and Hbs, and out of the possible genotypes only homozygous individuals for Hbs (HbsHbs) show the diseased phenotype. Heterozygous (HbAHbs) individuals appear apparently unaffected but they are carrier of the disease as there is 50% probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait.

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4. Phenylketonuria

This is also an autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. This phenylalanine is accumulated and converted into phenyl pyruvic acid and other derivatives. These are excreted through urine because of its poor absorption by kidney.

5. Thalassemia

It is an inherited autosomal codominant blood disease. The genetic defect results in reduced rate of synthesis of one of the globin chains that make up haemoglobin. Reduced synthesis of one of the globin chains can cause the formation of abnormal haemoglobin molecules, thus causing anaemia.

Chromosomal Disorders:

1. Turner’s syndrome: (XO female) – short stature; sterile (immature sex organs); often reduced mental abilities; about 1 in 2500 human female births.

2. Klinefelter’s syndrome: (XXY male) – often not detected until puberty, when female body characteristics develop; sterile; sometimes reduced mental abilities; testosterone shots can be used as a partial treatment; about 1 in 500 human male births

3. XYY Syndrome: (XYY male) – usually tall, with heavy acne; some correlation with mild mental retardation and with aggressiveness; usually still fertile; about 1 in 1000 human male births

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4. Triple X syndrome: (XXX female) – usually just like XX females, except for having 2 Barr bodies in somatic cells; HOWEVER, more likely to be sterile, and if fertile, more likely to have XXY and XXX children; about 1 in 1000 human female births

5. Down’s syndrome: (Trisomy in chromosome 21)  the only autosomal trisomy condition in humans that allows an appreciable number of individuals to survive to adulthood. Found in about 1 in 750 live births. Traits include abnormal facial appearance, high likelihood of mental retardation (degree varies considerably), and increased likelihood of developing leukemia and Alzheimer’s disease.

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