1. Biology Unit Four C – Chromosomes and Phenotype
Mrs. Pearson’s First Semester Biology
*For those watching by recording, watch for questions on the slides throughout
the presentation. me the answers to the questions within two days of the
lesson being presented in order to receive credit for watching the recording.

2. Lesson Objectives
Discuss nature of sex linked traits
Discuss factors affecting the expression of phenotypes

3. Getting Rave Reviews…
Phenotype
Genotype
Recessive
Dominant
Heterozygote
Homozygote
Allele

4. Word of the Day!
Autosomal
Any chromosome that is NOT one for determining whether the organism is male or female.

5. Two copies of each autosomal gene affect phenotype – How do we know this?
Mendel studied autosomal gene traits, like hair texture.
If a gene is not autosomal, what is it?

6. Mendel’s rules of inheritance apply to autosomal genetic disorders.
A heterozygote for a recessive disorder is a carrier. Why?
Disorders caused by dominant alleles are uncommon.
(dominant)
**Those watching by recording, include in your why it is uncommon to have a dominant allele that causes a disorder.

7. Males and females can differ in sex-linked traits.
Genes on sex chromosomes are called sex-linked genes.
Y chromosome genes in mammals are responsible for male characteristics.
X chromosome genes in mammals affect many traits – not just female traits.

8. Male mammals have an XY genotype.
All of a male’s sex-linked genes are expressed – Why?
Males have no second copies of sex-linked genes.
**Those watching by recording, include in your why all of a male’s sex linked genes are expressed.

9. Female mammals have an XX genotype.
Expression of sex-linked genes is similar to autosomal genes in females – why?
X chromosome inactivation randomly “turns off” one X chromosome.

10. Phenotype can depend on interactions of alleles.
In incomplete dominance, neither allele is completely dominant nor completely recessive – what might a heterozygous individual look like?
Heterozygous phenotype is intermediate between the two homozygous phenotypes

11. Codominant alleles will both be completely expressed
Codominant alleles will both be completely expressed. What might that look like?
Codominant alleles are neither dominant nor recessive.
The ABO blood types result from codominant alleles.
Many genes have more than two alleles.

12. Many genes may interact to produce one trait.
Polygenic traits are produced by two or more genes.
Order of dominance: brown > green > blue.

13. Phenotype is a combination of genotype and environment.
The environment interacts with genotype – think of a way the environment may affect phenotype.
Phenotype is a combination of genotype and environment.
The sex of sea turtles depends on both genes and the environment
Height is an example of a phenotype strongly affected by the environment.
**Those watching by recording, include in your why height is strongly affected by environment.

14. Review
Two copies of each autosomal gene affect phenotype
Males and females can differ in sex-linked traits\
Phenotype can depend on interactions of alleles
In incomplete dominance, neither allele is completely dominant nor completely recessive
Codominant alleles will both be completely expressed
Many genes may interact to produce one trait
The environment interacts with genotype

15. Questions?