NEET Biology Notes Genetics Sex-determination
Sex-determination
Sex-determination
In dioecious species (separate sexes), there are ‘several means to determine sex, The chromosomes involved in sex determination are called sex chromosomes.
All other chromosomes are called autdsomal chromosomes or autosomes. Although sex chromosomes provide the most common means of sex determination, it is not the only mechanism.
- In bees, males are haploid (n), while females are diploid (2n).
- Sex may be determined by a single allele or multiple alleles as in some wasps.
- By environmental factors as in some turtles, these have indeterminate genetic sex-determining mechanisms.
There are five basic types of chromosomal mechanisms:
- In humans XX-XY system, In which females are homomorphic XX and males are heteromorphic XY. This is found in mammals including humans and some insects including Drosophila.
- In birds ZW-ZZ system, In which females are heteromorphic ZW and males are homomorphic ZZ. This occurs in birds, some fishes and moths. It is essentially the opposite of XY in mammals.
- The characters or traits carried on sex.
- chromosomes are called sex-linked traits and their inheritance is called sex-linked inheritance. First of all TH Morgan (1910) observed sex-linked inheritance in Drosophila (fruit fly). In humans, colour blindness (red-green blindness) and haemophilia (Bleeder’s disease) are important sex-linked (X-linked) diseases.
- The genes located exclusively on X-chromosomes are called X-linked and the genes that occur only on Y-chromosome are called holandric genes.
Sex linked inheritance
whereas the genes preseht on homologous regions in sex chromosomes are called XY-linked genes.
The two sex-linked inheritance as dominance and recessive are given below:
- Sex-Linked Dominance
No generations are skipped. Affected males must come from affected mothers. All the daughters, but none of the sons, of an affected father are affected. Approximately half of an affected female’s sons and – daughters are affected. - Sex-Linked Recessive
Males are most affected. Affected males have carrier mother, who are known to have affected brothers, father or maternal uncles. Affected females come from affected father and carrier mother. Affected female’s sons must be affected. Approximately half the sons of carrier females should be affected.
Autosomal Linked Inheritance
The two autosome linked inheritance as dominant inheritance and recessive inheritance are given below:
- Autosomal dominant inheritance It is caused by dominant autosomal gene. In autosomal dominant inheritance if the trait is rare, most matings are between heterozygotes (affected) and homozygous (unaffected) recessives, which lead to predominantly 1 : 1 phenotypic ratios for all children, regardless of sex. The dominant phenotype should appear in every generation, e.g. Achondroplasia, polydactyly, brachydactyly, Huntington’s chorea, phenylthiocarbamide testing, aniridia, etc.
- Autosomal recessive inheritance It is caused by recessive autosomal gene present in homologous condition both parents are affected, all children will be affected. Often found in consanguineous marriages. Most affected children have normal parents, e.g. alkaptonuria, albinism, Tay-sach’s disease, gaucher’s disease, sickle-cell anaemia, thalassemia and cystic fibrosis, etc.
Pedigree Analysis
The analysis of traits in several generations of a family is called pedigree analysis. In the pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations. It is a strong tool to trace the inheritance of a specific trait, abnormality or disease.
Some specific symbols are used to show pedigree in a pedigree chart:
Mutation
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.
Depending upon the cause, mutations are of three types:
- Genomatic mutations These are caused due to the changes in chromosome number.
They incliide polyploidy (the presence of more than two sets of chromosomes) and aneuploidy (presence of fewer or extrachromosomes than the mammal). - Chromosomal mutations These are caused due to thechanges in number and arrangement of genes in the chromosomes. They include deficiency, deletion, inversion, duplication and translocation.
- Gene mutations These are changes in gene structure due to alteration in nucleotide number, type and sequence.
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 Mena.nan disorders are haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria, thalassemia, etc.
- 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 antjfraemophilic globulin (factor-VIII).
About 4/5 cases of haemophilia belong to haemophilia-A. Haemophilia-B or Christmas disease results from a defect in Plasma Thromboplastic Component (PTC or factor-IX). - Colour Blindness
It is a condition, in which certain colours (green or red) 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 repessive gene and is thus more common in males than females.
The symbols needed to follow this allele through a Punnett square are: - X = X-chromosome carrying the normal
(dominant) allele. - Xc = X-chromosome carrying the defective .
(recessive) allele. - Y = Y-chromosome, with no allele for this gene on it.
- Sickle-cell Anaemia
This is an autosome-linked recessive trait that can be transmitted from parents to the offspring when, both the partners w arrier or affected for the gene. The disease is cou xled by a single pair of allele, HbA and Hbs, 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. - 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. - Thalassemia
It is an inherited autosomal codominant blood disease. In thalassemia, 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
The chromosomal disorders are caused due to the absence or excess or abnormal arrangement of one or more chromosomes. The failure of segregation of chromatids during cell division cycle results in the gain or loss of a chrombsomes called aneuploidy. The failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organisms. This phenomenon is called polyploidy. The condition is commonly seen in plants: –
Sometimes, either an additional copy of a chromosome may be included in a person or may lack one of one pair of chromosomes. These situations are called as trisomy or monosomy of a chromosome, respectively.
- Down’s Syndrome
In this disorder, the presence of an additional copy of the chromosome number 21 (trisomy of 21) occurs.
The affected individual is short-statured with small round head, furrowed tongue and partially open mouth. Palm is broad wiSi characteristic palm crease. Physical psychomotor aii’ff hiental development is retarted. - Klinefelter’s Syndrome
This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47 (XXY). Such individuals have overall masculine development, however, the feminine development (development of breast, i.e. gynaecomastia) is also expressed. Such individuals are sterile. - Turner’s Syndrome
This disorder is caused due to the absence of one of the X-chromosomes, i.e. 45 with XO.
These females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters. - Cri-du-Chat Syndrome
It is caused due to a deletion in the short arm of chromosome number 5. The affected newborn cries in a high pitched sound like mewing of a cat.