genetics bio101 lecture

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