Extending Mendelian genetics Ch. 7 Extending Mendelian genetics

Phenotypes and complex inheritance Ch 7.1 and 7.2 Phenotypes and complex inheritance

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

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

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

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

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

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

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?

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

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

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

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?

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?

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

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

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?

X-Linked Dominant

Homework: For tomorrow: 7.2 # 3

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

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

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

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

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

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

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