Chromosomal sex determination
Inheritance of signs of adhesion to the floor
Despite numerous attempts, it was not possible to explain why the number of males and females in dioecious species is approximately the same and how the same pair of parents gives offspring of different sexes.
Conventionally, all methods for determining the sex of an organism can be divided into 3 options:
A) before fertilization – rotten , in the process of egg maturation during oogenesis.
B) after fertilization – under the influence of the external environment ” epigamous “
C) in the process of fertilization – ” syngamous “.
Note : Daphnia females (small crustaceans), under favorable conditions, produce similar female offspring. With the deterioration of the conditions of existence, both females and males can develop from ordinary parthenogenetic eggs. Under adverse conditions, females produce haploid eggs. After fertilization, the females lay winter diploid eggs, from which, when favorable conditions occur, parthenogenetic females again develop. In some organisms, sex determination occurs after fertilization, depending on environmental influences. The most common type of sex determination in dioecious species at the time of fertilization and depends on the set of sex chromosomes. This is chromosomal sex determination.
The sex of an organism is a set of properties and traits that ensure reproduction and transmission of hereditary information in a generation.
2 Signs of gender:
– primary sexual
– secondary sex
– limited by gender
– sex-linked traits.
Primary sexual characteristics include the presence of specific genital organs and gonads. The symbol of women is the ovaries, the symbol of men is seeds. Their development is directly dependent on the combination of sex chromosomes “XY”
Secondary sexual characteristics are phenotypic differences between the female and male sexes, which are formed in the process of ontogenesis. At the onset of puberty: body type, development of the mammary glands, voice timbre.
Sex- limited traits are signs that are different in different sexes or only appear in one sex.
For example: the egg-laying gene appears only in females, the milk-production gene appears only in cows, horned rams and hornless sheep.
Chromosomal sex determination.
The chromosome set of males and females in most dioecious organisms is not the same. Drosophila body cells have 4 pairs of chromosomes: 3 pairs are autosomes (chromosomes are the same in both sexes (male and female), identical in males and females, and 1 pair of chromosomes, different in male individuals and female.These chromosomes are called sex , which are responsible for the inheritance of sex.
The sex chromosomes of females are the same and are conventionally referred to as X chromosomes. In males, they are different: one chromosome is X, and the other is a Y chromosome. Accordingly, the chromosome set of females is XX, and males are XY.
/ The eggs of Drosophila females are identical in chromosome set, each of them contains three autosomes and one X chromosome. Spermatozoa have the same set of autosomes, but differ in sex chromosomes: one half of the spermatozoa carries the X chromosome, the other half has the Y chromosome. An egg is equally likely to be fertilized by a sperm with an X or Y chromosome.
In humans, the chromosomal mechanism for determining sex is the same as in Drosophila. The diploid number of human chromosomes is 46, of which 22 pairs of autosomes and 1 pair of sex chromosomes. Women have two X chromosomes, men have X and Y chromosomes. Females form one type of gamete, and males form two types of gametes. The female sex is homogametic , while the male sex is heterogametic .
In nature, the opposite type of sex determination is also found. For example, in birds, reptiles, the male sex is homogametic, and the female sex is heterogametic.
The sex of a person is determined at the time of fertilization and depends on the combination of sex chromosomes. In dioecious organisms, the ratio between males and females approaches a 1:1 ratio.
Inheritance of sex-linked traits
Traits inherited through the sex chromosomes are called sex-linked .
The Y chromosome is small in size and does not contain alleles of many genes located on the X chromosome, therefore, in the heterogametic sex, most of the genes are localized on the X chromosome and are in a homozygous state, that is, they do not have an allelic pair, and the traits controlled by them appear phenotypically even if if the gene is represented by one allele.
In humans, sex-linked traits include color blindness (color blindness), hemophilia, and others.
In humans, the Y chromosome is passed from father to son, the traits that are inherited are called hollandic . These signs include the intensive development of hair along the edge of the auricle.
Hemophilia, color blindness, optic nerve atrophy, muscular dystrophy (recessive traits) are inherited through the X chromosome.
One form of hemophilia is caused by a recessive gene (h) linked to the X chromosome. The H gene determines normal blood clotting. Heterozygous women (X n X h ) – carriers of hemophilia are phenotypically healthy. They can pass on the X h gene to their daughters, but since the daughters receive the X chromosome with the H gene anyway from a healthy father, they will all be healthy, and among the sons there may be hemophiliacs.
Females can be homozygous and heterozygous for alleles localized on the X chromosome, and they have recessive alleles only in the homozygous state. When writing the scheme for the transmission of sex-linked traits in genetic formulas, along with the symbol, sex chromosomes are indicated.
X n – dominant, normal blood clotting
X h – recessive hemophilia
X D – dominance, color of vision is normal
X d – recessive trait, color blindness.
The so-called “virgin reproduction”, one of the forms of sexual reproduction of organisms, in which female germ cells develop in an adult organism without fertilization. Although parthenogenetic reproduction does not involve the fusion of male and female gametes, parthenogenesis is still considered sexual reproduction, since the organism develops from a germ cell. It is believed that parthenogenesis arose in the process of evolution of dioecious forms.