1. Extending Mendelian genetics
Ch. 7
Extending Mendelian genetics

2. Phenotypes and complex inheritance
Ch 7.1 and 7.2
Phenotypes and complex inheritance

3. Lesson Objectives At the end of this lesson students will be able to…
Explain the effect alleles have on phenotype in terms of genetic disorders and sex linked traits
Explain why incomplete dominance, co-dominance, and polygenic traits are considered more complex than Mendel genetics
List some ways the environment can affect the expression of genes

4. Chromosomes and Phenotype
We know:
Organisms get one copy of each chromosome/gene from each parent – this results in pairs of genes – Diploid
Genes affect the phenotype – the physical expression of genetics
The expression of traits depends on the allele – dominant or recessive

5. Types of Traits What determines the inheritance of “neutral” traits?
What if the traits aren’t so neutral?

6. Inheritance of Positive Traits
With genetics, the more offspring you are able to produce – the more your genetics get passed on
Some traits increase successful reproduction which means that particular gene gets passed on more often – it may even become amplified
Ex: The peacock

7. Inheritance of Positive Traits
In peacocks, feather color and size determines if a female will mate with a male
The most colorful males get to produce the most offspring and pass on their genetics

8. Inheritance of Positive Traits
Another example of certain traits increasing survival and reproduction is snail shell thickness
In populations where snails are getting eaten by crabs, the snails with stronger shells were surviving
These genetics get passed on and eventually the thin shells go away

9. Negative Trait Inheritance
On the other hand, certain alleles can be negative and will decrease the success of reproduction
This will decrease how often the genes are passed on therefore making the trait less common
If this is true, why do we still have so many genetic diseases?

10. Genetic Disorders
Genetic disorders are a category of health complications that are determined by an organism’s genes
These disorders may not show up until later in life but they are always in the DNA code

11. Recessive Autosomal Disorders
Some genetic disorders are recessive so people can carry an allele for the disorder without even knowing
These people are called “CARRIERS”
2 carriers have a 25% chance of having an affected child

12. Recessive Autosomal Disorders
More than 1,000 recessive genetic disorders have been identified. Some Examples are:
Cystic fibrosis
Phenylketonuria (PKU)
Albinism
*Sickle-cell anemia*
Tay-Sachs disease
Growth hormone deficiency

13. Try it:
If you have a carrier mom (Dd) and a normal dad (DD), can any of the children end up with the disease?
Will any of them be carriers?

14. Dominant Autosomal Disorders
Some genetic disorders are determined by the dominant allele
1 affected parent + 1 normal parent have a 50/50 chance of having affected children
Can someone be a CARRIER for a dominant disorder?

15. Dominant Autosomal Disorders
LESS common than recessive genetic disorders
Why?
Examples: Huntington’s disease, achrondroplasia, and Marfan syndrome

16. Sex Linked Traits
These are traits with genes on the sex chromosomes (X or Y)
These genes can cause neutral traits such as hair color in cats, or they can cause genetic disorders
Since males and females have a different combination of sex chromosomes, they are affected differently

17. X-Linked Recessive Disorders
A carrier mother + normal father will have a 25% chance of having an affected male and 25% chance of having a carrier female
Why can’t males be carriers?

18. X-Linked Dominant

19. Homework:
For tomorrow:
7.2 # 3

20. Complex Inheritance
Unfortunately, not all traits are as easily understood as the ones Mendel was looking at…

21. Incomplete Dominance
Incomplete dominance occurs when one alleles isn’t COMPLETELY dominant over the other
So instead of having two phenotypes – dominant and recessive – you would get an intermediate phenotype from the heterozygous combination
This is very common with coloring in various organisms

22. Incomplete Dominance
The heterozygous combination of alleles shows a third phenotype instead of just dominant or recessive
Flowers on right: red dominant, pink heterozygote, white recessive
See betta fish on pg. 193

23. Codominance
With codominance, both traits completely show through – notice the cow isn’t a light brown, it shows white fur AND dark brown at the same time

24. Polygenic Traits
So far we’ve talked about traits that are determined by a single gene
Traits can also be determined by MULTIPLE genes that might not even be on the same chromosome
Eye color
online

25. Environment: Chemical
The environment can also influence how genes are expressed
The presence of chemicals during growth can cause genes to be produced too much or not enough
Premie babies can be born blind if given too much oxygen
Frogs can grow extra limbs if exposed to pollution
Identical twins can get a different amount of nutrition in the uterus causing slightly different development

26. Environment: Temperature
The temperature and amount of light in the environment can also influence the expression of genes
Remember the sea turtle and crocadile eggs??
Himalayan rabbits develop different color if raised in different temps – white from warmer, black from cooler