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Pa#erns of Inheritance
I. Mendel’s Laws I. Mendel and his pea plant observa9ons
II. Law of segrega9on and inheritance of a single trait III. Law of independent assortment and inheritance of
mul9ple traits. II. Varia9ons on Mendel’s Laws
I. WHY and HOW inheritance of most traits do NOT follow Mendel’s Laws
III. The Chromosomal Basis of Inheritance I. Where are genes located and what does meiosis have to
do with it? IV. Sex Chromosomes and Sex-‐Linked Genes
I. Why do sex-‐chromosome-‐linked disorders affect mostly males?
Meiosis Vs. Mitosis
Homologous chromosomes carry different versions of the same genes
“Crossing over” makes chimeric chromosomes
Why DO children resemble their parents?
Early ideas for how traits are inherited
• 19th century “Blending” hypothesis
Does “Blending” hypothesis apply to biology?
x
Gregor Mendel, monk, gene9cist.
Gregor Mendel and his pea
plants
What did Mendel OBSERVE?
From these OBSERVATIONS came four HYPOTHESES!
1. There are different forms of “GENES” that account for different inherited characteristics.
2. For each characteristic, an organism inherits 2 ALLELES, one from each parent.
3. If the two alleles are different, the one that determines appearance is the DOMINANT ALLELE, the other is the RECESSIVE ALLELE. 4. A sperm or an egg cell carries only one form of the allele because the pair of alleles gets split up during meiosis. This is the LAW OF SEGREGATION.
What do these HYPOTHESES mean?
Where are genes found?
Test Your Knowledge
• With your neighbor, complete the following Punnet Square:
Mother: Ww – Heterozygous for widow’s peak hairline
Father: ww-‐ Homozygous for straight hairline
Mul9ple characteris9cs in pea seeds
How did Mendel determine what offspring peas would look like (phenotype) when he crossed peas with MULTIPLE characteristics?!
Mendel’s Law of Independent Assortment
• Each pair of alleles SEGREGATES independently of OTHER pairs of alleles.
n In other words: The different versions of each gene on homologous chromosomes will assort independently of other genes.
n Convince yourself by drawing a chromosome with 2 different genes and take them through meiosis express.
Test your understanding
Mother’s genotype: AaBb
Father’s genotype: AAbb
How to use a testcross to determine an unknown genotype
Mendel’s Laws are Governed by Probability (chance)
How do we know if diseases we have in our families will be passed down to our children? Pedigrees.
Test your understanding
Discuss with your neighbor:
The male child in this family pedigree is affected with a geneDc disorder.
If the genotype of the diseased son is vv, what is the genotype of his parents?
What COULD the genotype of his siblings be?
Many gene9c disorders are controlled by a single gene
Achondroplasia- caused by a dominant allele for dwarfism.
Cystic Fibrosis- caused by a recessive allele carried by BOTH parents.
A Helpful Illustra9on
The big “BUT”: Non-‐Mendelian Gene9cs
• Many traits are NOT inherited in a “SIMPLE MENDELIAN” fashion.
• Examples: – Skin color – Blood type – Height – Many, many, many other traits.
Non-‐Mendelian inheritance: Case 1
What kind of dominance might we call this?
INCOMPLETE DOMINANCE
Why is pea colora9on different than carna9on colora9on?
• Different sets of genes control the colors of these two species of flowers.
Non-‐Mendelian Inheritance Case #2: MULTIPLE ALLELES
• What kind of dominance is this where we see BOTH traits?
CO-DOMINANCE
What does the blood test actually look like?
Non-‐Mendelian inheritance Case #3: One gene can affect many characteris9cs
Pleiotropy
Sickle Cells and Normal Cells
Non-‐Mendelian Inheritance Case #4: one trait controlled by many genes
Polygenic inheritance
Non-‐Mendelian Inheritance Reviewed!
The environment can also affect physical characteris9cs
Tanorexia and it’s consequences.
Other side effects of anabolic steroids include acne, reduced tesDcle size.
What do Mendel’s Laws have to do with Chromosomes?
Some genes are so close together that they tend to be inherited
together We call these ‘LINKED GENES’
CROSSING OVER creates new combina9ons of alleles in gametes
Chromosomes determine sex in many species, including our own
Some genes are linked to sex chromosomes
These genes are called “SEX-LINKED” genes.
Sex-‐linked traits show unusual inheritance pa#erns
Sex-‐linked disorders affect mostly males. : (
Hemophilia in the Russian royal family. The moral of the story: DON’T MARRY YOUR FIRST COUSIN.
If you can’t read this, I’m sorry, but you cannot fly for the air force.
Fragile X Syndrome • Caused by a muta9on (duplica9on) within the X chromosome