6.3 Lesson 6.3: Human Genetics.
Lesson Objectives
- List the two types of chromosomes in the human genome.
- Predict patterns of inheritance for traits located on the sex chromosomes.
- Describe how some common human genetic disorders are inherited.
- Explain how changes in chromosomes can cause disorders in humans.
Check Your Understanding
- How many alleles does an individual have for each gene/trait?
- How do we predict the probability of traits being passed on to the next generation?
- What do we call complexes of DNA wound around proteins that pass on genetic infor-
mation to the next generation of cells?
Introduction
You might know someone who was born with a genetic disorder, such as cystic fibrosis
or Down syndrome. And you might have wondered how someone inherits these types of
disorders. It all goes back to Mendel! Mendel’s rules laid the foundation for understanding
the genetics of all organisms, including humans. We can apply Mendel’s rules to describe
how many human traits and genetic disorders are inherited. Some disorders are caused by
a recessive allele, while other disorders are caused by a single dominant allele. Therefore,
we can draw a Punnett square to predict the number of offspring that may be affected with
these diseases, just like we predicted for other traits in the previous lessons. Since Mendel’s
time, we have also expanded our knowledge of inheritance and understand that genes are
located on chromosomes. Now we can now explain special inheritance patterns that don’t
fit Mendel’s rules.
Sex-linked Inheritance
What determines if a baby is a boy or a girl? Recall that you have 23 pairs of chromosomes,
one pair of which are the sex chromosomes. Everyone has two sex chromosomes, X or Y, that
determine our sex. Females have two X chromosomes, while males have one Y chromosome
andoneXchromosome. Soifa babyinheritsanX fromthefatherand anXfrom themother,
it will be a girl. If the father’s sperm carries the Y chromosome, it will be a boy. Notice
that a mother can only pass on an X chromosome, so the sex of the baby is determined by
the father. The father has a 50 percent chance of passing on the Y or X chromosome, hence
it is a 50 percent chance whether a child will be a boy or a girl.