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Unlock This CourseSex determination is the biological process that decides whether an organism develops as male or female. In humans and many other animals, this happens through special chromosomes called sex chromosomes. Understanding how this works helps us predict the chances of having male or female offspring and explains why certain traits are more common in one sex than the other.
Key Definitions:
Males have XY sex chromosomes. The Y chromosome is much smaller than the X and carries fewer genes. Males only need one copy of any gene on the X chromosome to express that trait, making them more likely to show X-linked recessive conditions.
Females have XX sex chromosomes. Having two X chromosomes means females need two copies of a recessive allele to express X-linked recessive traits. This gives them some protection against harmful recessive genes on the X chromosome.
Humans have 23 pairs of chromosomes, making 46 in total. Twenty-two pairs are autosomes (the same in males and females) and one pair consists of sex chromosomes. The sex chromosomes determine whether you develop as male or female.
Sex is determined by which sex chromosome comes from the father. Mothers always contribute an X chromosome to their offspring, whilst fathers can contribute either an X or a Y chromosome. This creates a 50:50 chance of having male or female offspring.
Always contributes an X chromosome. This is because females only have X chromosomes (XX) to give.
Can contribute either X or Y. Males have XY, so they determine the sex of the offspring.
50% chance of XX (female) and 50% chance of XY (male) in each pregnancy.
Parents: Female (XX) × Male (XY)
Gametes: X from female, X or Y from male
Offspring: XX (female) : XY (male)
Ratio: 1:1 or 50% female : 50% male
Some genes are located on the sex chromosomes, particularly the X chromosome. These are called sex-linked genes. Because males only have one X chromosome, they're more likely to express recessive traits carried on the X chromosome.
Several important human conditions are caused by recessive alleles on the X chromosome. These affect males much more frequently than females because males only need one copy of the recessive allele to express the condition.
Red-green colour blindness affects about 8% of men but only 0.5% of women. It's caused by a recessive allele on the X chromosome that affects how the eye detects certain colours.
This blood clotting disorder is also X-linked recessive. People with haemophilia have difficulty forming blood clots, leading to excessive bleeding from even minor injuries.
When working with sex-linked traits, we need to show the sex chromosomes in our genetic diagrams. We use superscript letters to show which alleles are carried on each X chromosome.
Let's use 'C' for normal colour vision (dominant) and 'c' for colour blindness (recessive). Remember, these alleles are only found on the X chromosome.
Mother: X^C X^c (carrier - normal vision)
Father: X^C Y (normal vision)
Offspring possibilities:
- X^C X^C (normal female)
- X^C X^c (carrier female)
- X^C Y (normal male)
- X^c Y (colour blind male)
Results: 25% chance of colour blind male, 25% chance of carrier female
Not all organisms use the XY system for sex determination. Different species have evolved various methods to determine sex, each suited to their particular lifestyle and reproduction needs.
Use ZW system - females are ZW (like males in XY system) and males are ZZ (like females in XY system).
Use haplodiploid system - females develop from fertilised eggs (diploid) whilst males develop from unfertilised eggs (haploid).
Can change sex during their lifetime based on environmental conditions or social factors - called sequential hermaphroditism.
Some reptiles, including many turtles and crocodiles, don't have sex chromosomes at all. Instead, the temperature during egg incubation determines whether the offspring develop as male or female.
In many turtle species, cooler temperatures (below 29°C) produce mainly males, whilst warmer temperatures (above 29°C) produce mainly females. This system might make these species vulnerable to climate change, as rising temperatures could lead to mostly female offspring.
Understanding sex determination and sex-linked inheritance has important practical applications in medicine, agriculture and conservation.
Genetic counsellors use knowledge of sex-linked inheritance to help families understand their risk of passing on conditions like haemophilia or colour blindness to their children.
Farmers can use sex determination knowledge to breed animals with desired traits. For example, in dairy farming, female calves are often more valuable than males.
Sex determination is a fundamental biological process that affects inheritance patterns. The XY system used by humans creates equal chances of male and female offspring, but sex-linked genes on the X chromosome create different inheritance patterns for males and females. Males are more likely to express X-linked recessive traits because they only have one X chromosome, whilst females need two copies of a recessive allele to express the same trait.
• Affected males cannot pass X-linked conditions to their sons (they give sons the Y chromosome)
• All daughters of affected males are carriers (they receive the father's X chromosome)
• Carrier mothers have a 50% chance of passing the condition to each son
• The condition typically 'skips generations' in family pedigrees