 Identify some patterns of inheritance in humans.  Describe the function of the sex chromosomes.  Explain the relationship between genes and the environment.

 Some human traits are controlled by single genes with two alleles, and others by single genes with multiple alleles. Still others are controlled by many genes that act together.

 Several human traits are controlled by a single gene with one dominant allele and one recessive allele.  These traits have two distinctly different phenotypes.  This type of inheritance includes the following traits: attached/free earlobes and a widow’s peak.

 Some human traits are controlled by a single gene that has more than two alleles (called multiple alleles).  Multiple alleles is three or more forms of a gene that code for a single trait.  Even though a gene can have multiple alleles, a person can only carry two of those alleles because chromosomes exist in pairs. Each chromosome in a pair only carries one allele for each gene.

 Human blood type is controlled by a gene with multiple alleles:  There are four main blood types: A, B, AB, and O. Three alleles control the inheritance of blood types. Blood type:Combination of Alleles: AI A I A or I A i BI B I B or I B i ABIAIBIAIB Oii

 The allele for blood types A and B are codominant.  The alleles for blood type O is recessive.  When two codominant alleles are inherited, neither allele is masked, thus a person who inherits both an A allele and a B allele, has the AB blood type.

 Some human traits show a large number of phenotypes because the traits are controlled by many genes.  These genes act together as a group to produce a single trait.  At least four genes control height in humans, so there are many possible combinations of genes and alleles.

 Skin, eye and hair color are other human traits that are controlled by multiple alleles.

 The sex chromosomes are one of the 23 pairs of chromosomes in each body cell.  The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits.

 The sex chromosomes are the only chromosome pair that do not always match.  If you are a girl, your sex chromosomes match: XX.  If you are boy, your sex chromosomes do not match: XY.

 All eggs carry one X chromosome because all females have two X chromosomes.  Each sperm has either an X or a Y chromosome (but not both) because all males have an X and a Y chromosome.  Half of a male’s sperm will have an X chromosome and half will have a Y chromosome.

 If a sperm with an X chromosome fertilizes an egg, the baby will be a girl.  If a sperm with a Y chromosome fertilizes an egg, the baby will be a boy.

 The genes for some traits are carried on the sex chromosomes.  Genes on the X and Y chromosomes are often called sex-linked genes because their alleles are passed from parent to child on a sex chromosomes.  Sex linked traits include red-green color blindness. A person with this trait cannot distinguish between red and green.

 Unlike most chromosome pairs, the X and Y chromosomes have different genes.  Most of the genes on the X chromosome are not on the Y chromosome, so an allele on an X chromosome may not have a corresponding allele on the Y chromosome.

 Like other genes, sex-linked genes can have dominant and recessive alleles.  In females, a dominant allele on one X chromosome will mask a recessive allele on the other X-chromosome so the organism will not be affected in her phenotype.

 In males, there is no matching allele on the Y chromosome to mask the allele on the X chromosome, so as a result, any allele on the X chromosome (even a recessive one) will produce the trait in a male who inherits it.  Males are much more likely than females to have a sex-linked trait that is controlled by a recessive allele

 Colorblindness is a trait controlled by a recessive allele on the X chromosome.  Many more males than females have red- green colorblindness because the female would have to receive recessive alleles from both mom and dad.  Males only have to receive a recessive allele from mom in order to be red-green colorblind.

 Many of an organism’s characteristics are determined by an interaction between genes and the environment.  Genes are not the factors that influence height.  Someone’s diet can affect a person’s height.  A diet lacking in protein, certain minerals, or certain vitamins can prevent someone from growing as tall as might be possible.