Sex-determination and Sex-linked Inheritance.pptx

The sexually reproducing organisms are classified into two types such as monoecious (hermaphrodite) and dioecious. In monoecious organisms, both male and female gametes (sex cells) are produced by a single individual. The organisms in which both male and female gametes are produced by different individuals are called dioecious. Living organisms, with a very few exceptions, are differentiated into male and female individuals. The sexes of the individuals are genetically determined.
The biological system that determines the development of sexual characteristics in an organism is called sex determination.
There are two different systems of sex determination- Chromosomal sex determination and Non-genetic sex determination.

1. Sem . V , Paper X Genetics Dr. Seema A. Gaikwad Dept. of Botany Vidnyan Mahavidyalaya , Sangola

2. Unit 3 Sex-determination and Sex-linked Inheritance Sex determination is the natural event by which an individual of a dioecious species becomes male or female. The mechanism by which sex is established is termed sex determination. We define the sex of an individual organism in reference to its phenotype. Sometimes an individual organism has chromosomes or genes that are normally associated with one sex but a morphology corresponding to the opposite sex.

3. Unit 3- Mechanism of Sex-determination Sexual Reproduction is the formation of offspring that are genetically distinct from their parents; most often, two parents contribute to their offspring and the genes are assorted into new combinations through meiosis. Among most eukaryotes, sexual reproduction consists of two processes that lead to an alternation of haploid and diploid cells; meiosis produces haploid gametes(spores in plants), and fertilization produces diploid zygotes.

4. Unit 3 Mechanism of Sex-determination The term Sex refers to sexual phenotype. Most organisms have only two sexual phenotypes: male and female. The fundamental difference between males and females is gamete size: males produce small gametes; female produces relatively large gametes.

5. Unit 3 Mechanism of Sex-determination Sex determination mechanism include- Chromosomal determination mechanism Genetically controlled sex determination H ormonally controlled sex determination Environmentally controlled sex determination

6. Unit 3 - Sex-determination and Sex-linked Inheritance Chromosomal Sex determination: Sex determination is associated with sex chromosomes that are different between male and female individuals. Many species have chromosomal sex- determining systems. As Stevens and Wilson found for insects, sex in many organisms is determined by a pair of chromosomes, the sex chromosomes, which differ between males and female. Chromosomal determination is found in, XX/XO mechanism XX/XY mechanism ZW/ZZ mechanism

7. Unit 3 Mechanism of Sex-determination Genetic sex determination: Sex is determined at fertilization by the combination of genes that the zygote receives. Hormonal sex determination: It refers to the extent of the body coordination system and its effects. Environmental sex determination: In some species, sex is determined after fertilization by environmental factors like temperature, population size, or sex of others.

8. Autosomes and sex chromosomes During the cell division, chromatin in the nucleus shrinks to a thread like structures named chromosomes. Two major types of chromosomes can be found in eukaryotic cells. They are autosomes and sex chromosomes. Humans have 22 homologous pairs of autosomes and one pair of sex chromosomes. The  main difference  between autosomes and sex chromosomes is that  autosomes are involved in determining the somatic characters of an individual and sex chromosomes are involved in determining the sex and the sex-related hormonal traits .

9. Autosomes and sex chromosomes What are Autosomes? : Non-sex chromosomes which determine the trait of an organism is identified as autosomes. They are also known as somatic chromosomes since they determine the somatic characters of an individual. A genome mainly consists of autosomes. For example, human body contains 46 chromosomes within its genome and 44 chromosomes of them are autosomes. Autosomes exist as homologous pairs and 22 autosome pairs can be identified in the human genome . Both autosomal chromosomes contain the same genes, which are arranged in the same order. But an autosomal chromosome pair differs from other autosomal chromosome pairs within the same genome. These pairs are labeled from 1 to 22, according to the base pair sizes contained in each chromosome.

10. Autosomes and sex chromosomes What are Sex Chromosomes? Sex chromosomes are referred to as  allosomes . They determine the sex of an individual. The sex determination also happens in most animals and many plants. Humans have only 2 sex chromosomes in their genome which are labeled as X chromosome and Y chromosome. A female individual is determined by XX and a male individual is determined by XY. A female contains the same two copies of sex determining genes arranged in the same order in both X chromosomes (homomorphic). Therefore the sex chromosomes in a female are homologous to each other. In male’s, the two sex chromosomes contain different genes (heteromorphic).

11. Autosomes and sex chromosomes Sr. No. Autosomes Sex Chromosomes 1 Most of the chromosomes within a genome are autosomes A few of the chromosomes within a genome are sex chromosomes 2 The 22 pairs of autosomes are homologous in humans Female sex chromosomes (XX) are homologous (homomorphic) while male sex chromosomes (XY) are non-homologous (heteromorphic) 3 Since autosomes are homomorphic, the position of the centromere is identical Since the male sex chromosomes are heteromorphic, the position of the centromere is not identical. The position of the centromere in female sex chromosomes is identical 4 Autosomes contain the number of genes varying from 200 to 2000. Chromosome 1 which is the largest, carries about 2800 genes in humans X chromosome contains more than 300 genes while Y chromosome contains only a few genes since it is small in size 5 Autosomal disorders show Mendelian inheritance Sex-linked disorders show Non-Mendelian inheritance Difference Between Autosomes and Sex Chromosomes:

12. Autosomes and sex chromosomes

13. Sex chromosomes in Drosophila The fruit fly.  Drosophila melanogaster  has eight chromosomes. These chromosomes are present in the form of four homologous pairs. T.H. Morgan in 1911 found difference in the chromosomes of male and female Drosophila. The chromosomes of the three homologous pairs were similar in both of the sexes. But the fourth was  heteromorphic pair  and it had different structures. The female has two similar rod shaped X-chromosomes in the fourth pair. But male has one rod shaped X-chromosome but the other a morphologically different. J-shaped Y chromosome in the pair.

14. Sex chromosomes in Drosophila The  genome  of  D. melanogaster  contains four pairs of chromosomes – an X/Y pair, and three autosomes labeled 2, 3, and 4. The fourth chromosome is relatively very small and therefore often ignored, aside from its important  eyeless  gene.

15. Sex chromosomes in man Human body cells have 46 chromosomes: 22 homologous pairs of autosomes plus 2 sex chromosomes. In females, there is a pair of identical sex chromosomes called the  X chromosomes.  In males, there is a non-identical pair, consisting of one X and one Y. The  Y chromosome  is considerably shorter than the X. At meiosis in females, the two X chromosomes pair and segregate like autosomes so that each egg receives one X chromosome. Hence the female is said to be the homogametic sex. At meiosis in males, the X and the Y pair over a short region, which ensures that the X and Y separate so that half the sperm cells receive X and the other half receive Y. Therefore the male is called the heterogametic sex .

16. Sex chromosomes in man The human sex determination mechanism to a larger extent resembles XX - XY type of Drosophila. However, the Y chromosome contains male determining genes and it is the determiner of fertility and sex of male individual. Thus in human beings, the presence of Y chromosome determines maleness and its absence determines femaleness.

17. Balance concept of sex determination in Drosophila- Bridge’s Experiment

18. Balance concept of sex determination in Drosophila- Bridge’s Experiment

19. Sex linked inheritance in man Sex linked inheritance is the inheritance of the genes present on the sex chromosomes like X or Y or both XY, due to which some traits are transmitted to the next generation. This transmitted trait may be expressed phenotypically in one or both the sexes and sometimes in some generations only. During the inheritance of genes sex chromosomes carry not only the gene related to sex determination of the organism but also the genes related to some other characteristics. When any mutation occurs in this gene it leads to genetic disorders and are transmitted. Based on the linking and transmission of the genes sex-linked inheritance is classified into three types as I) X-Linked inheritance II)Y-Linked inheritance III)XY-Linked inheritance

20. X- linked inheritance This is the inheritance where the genes are carried from one generation to the next generation through X chromosome. In humans approximately fifty X – linked disorders have been identified.

21. Haemophilia

22. Haemophilia Haemophilia is an X – linked recessive disorder. It is of two types Haemophilia -A and Haemophilia -B. they are caused by lethal genes 1) Haemophilia – A is also known as royal disease as it was first identified in royal family of Queen Victoria and some other families. This is due to lack of blood protein called clotting factor VIII (AHG- anti haemophilic globulin). 2) Haemophilia - B is also known as bleeder’s disease because the person suffering with this disease has an inability of their blood to clot normally. This is due to the lack of blood protein called clotting factor IX (Christmas factor).

23. Haemophilia In the inheritance pattern if the female is possessing one recessive X chromosome for haemophilia (XH) she will not be affected by the disease but she will be a carrier to the disease. She will be affected only when she possess both X recessive chromosomes (XHXH). in case of males if he possess one X recessive chromosome for haemophilia (XH) he will be affected by the disease. He passes this character to his grandson through his daughter If the mother is normal and the father is haemophilic then all the daughter will be carriers and all the sons will be normal

24. Colour blindness Colour blindness is another common X-linked recessive disease in which the people have a defect in the cones of retina. Lack of colourable pigment in retinal cones is known as Deuteranopia or deutan colour blindness. Lack of erythrolable pigment for differentiating the red and green is known as Protanopia or protan colour blindness. Colour blindness is detected by Ishihara’s test introduced by Dr. Shinobu Ishihara The Ishihara Colour Test

25. Colour blindness Red-Green Colour Blindness Red-green colour blindness is a genetic disorder whereby an individual fail to discriminate between red and green hues This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome Red-green colour blindness can be diagnosed using the Ishihara colour test

26. Y - linked inheritance Genes that are present on the non-homologous region of the Y chromosome and are inherited from father to the grandson through son is known as Y-linked inheritance.

27. Holandric genes This pattern of inheritance is also known as Holandric Inheritance and the genes responsible for this inheritance are called Holandric genes. In humans holandric inheritance is signified by disease called Hypertrichosis or Ichthyosis hystrix gravis hypertrycosis . The disease is identified by the presence of excess hair on the body completely or restricted to a particular region like external pinna.

28. Holandric genes

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