Chapter 16: Simple Patterns of Inheritance

 

Summary

 

Introduction:

 

• Inheritance is the acquisition of traits by their transmission from parent to offspring.

 

 

16.1

 

• Hybridization – when two individuals with different characteristics are mated or crossed to each other

• The offspring are called hybrids

• Example: cross between a purple flowered plant and white flowered plant

• Mendel was intrigued by this and the consistency with which offspring of the crosses would have certain characteristics found in their parents

 

• Characters – different characteristics in appearance of an organism (such as height and seed color)

• Trait – indentifiable characteristic of an organism = variant (such as YELLOW seed color)

• Self fertilization – female gamete is fertilized by a male gamete from the same individual

• this is how Mendel's pea plants normally reproduced 

• True-breeding line – a variety that continues to exhibit the same trait after several generations of self-fertilization

• Cross-fertilization – fertilization that involves a male and female gamete from SEPARATE individuals

• Mendel did this in pea plants by cutting off the male part (stamen) in one plant and collecting pollen from the stamen of another plant and putting it into the female part (stigma) of the original plant

• Monohybrid cross – a cross where the experimenter only follows the variant of one character (also called a single-factor cross)

• parents are in the P generation
• offspring of the parents are F1 generation
• and then when those have offspring, it's the F2 generation

 

Mendel discovered there are two types of traits: 

• Dominant – the displayed trait – represented by an uppercase letter

 

• Recessive – masked by the presence of a dominant trait – lowercase letter

and he also discovered alleles

• Alleles – a variant form of a gene

• Genes - passed through generations, are the genetic determinants, “unit factors”

• Mendel’s law of segregation: two alleles of a gene separate (segregate) during the formation of eggs and sperm so that every gamete receives only one allele

• Genotype – genetic composition of an individual

• Homozygous – an individual with two identical alleles of a gene

• could be dominant (GG) or recessive (Gg) 

• Heterozygous – two different alleles of a gene

• one dominant and one recessive, but the dominant trait shows

• known as a carrier
  

   

• Phenotype – characteristics of an organism that are the result of the expression of its genes

• Gg and GG will show the dominant trait, and gg will show the recessive trait
  

   

• Punnett squares are used to predict the outcome of simple genetic crosses

• Dr. Weber doesn't like when we use these

 

 

 

 

 

16.2

 

• The idea of chromosomes carrying our genetic material first started to be developed during the early 1900s

• Researchers began to study mitosis, meiosis, and fertilization

• Chromosome theory of inheritance:

1. Chromosomes contain DNA – genetic material, genes are found here

2. Chromosomes are replicated and passed on

3. Each diploid cell has two sets of chromosomes – which are in homologous pairs – each set carries a full complement of genes

4. Meiosis involves one member of each chromosome pair segregating into one daughter nucleus

5. Gametes are haploid cells that combine to form a diploid cell during fertilization

 

• The physical location of a gene on a chromosome is called the gene’s locus

 

 

 

 

 

Section 16.3

 

• Human geneticists must rely on information from family trees aka pedigrees

• This is called pedigree analysis – an inherited trait is analyzed over the course of a few generations in one family

• May be less definitive due to the small size of the sampling, but can still provide important clues

• shaded shows affected individuals, half shaded shows carriers, unshaded shows no affect and no carry, and dotted shows sex linked

 

 

 

Many genes that play a part in genetic diseases exist in two forms – normal allele and an abnormal allele that has arisen by mutation – the disease comes from the mutation

 

Pedigree analysis shows whether the mutation is on the dominant or recessive allele

• you can tell it's dominant if a shaded and an unshaded individual produce a shaded child 
• recessive if two carriers or a carrier and an affected (shaded) individual can produce an affected child

 

Autosomes – paired chromosomes that human genes are found on

 

 

16.4

 

Sex chromosomes are a pair of chrosomes that are different in males and females

• found in most species with two sexes
• for mammals, males are XY and females are XX (X-Y system)
• for  certain insects, males are X and females are XX (X-O system)
• for birds, males are ZZ and females are ZW (Z-W system)
• for bees, males are haploid and females are diploid (haplodiploid system)
• males are the result of unfertilized eggs 

 

Let's talk about humans

• the X chromosome carries 10 times more genes than the Y chromosome
• Genes are sex linked if they're found on one but not the other
• more likely to be X linked than Y linked because X has more genes
• because males only have one X, they're hemizygous for an X linked gene
• some recessive diseases can be X-linked
• so they're more likely to show up in males because they only have one X chromosome
• females have two X chromosomes, so unless both of them have it, the normal one will mask the other

 

 

16.5

 

Mendelian inheritance - The inheritance patterns of genes that segregate and assort independently.

• Simple Mendelian inheritance - pair of alleles display a dominant/recessive relationship and are located on an autosome; dominant masks recessive
• X-linked inheritance -  genes located on X chromosome and show dominant/recessive relationship
• Incomplete dominance - the heterozygote's phenotype is a blend between the dominant homozygote and recessive phenotypes (like a pink flower from crossing a red and white one)
• Codominance - both alleles of a heterozygote can be expressed at the same time
• Sex-influenced inheritance - an allele that's dominant in one sex and recessive in the other (like baldness in males)

 

 

Alleles can be mutated (and called mutant alleles) but it's rare.

 

Phenotypes can also be influenced by the environment (norm of reaction)

 

 

 

 

16.6

 

Beginning talks about what probability is. If you don't know, repeat elementary school math immediately.

 

Product Rule - the probability that two or more independent events will occur is equal to the product of their individual probabilities.

 

EXAMPLE: If a disease is recessive, and two carries mate, they have a 1/4 chance of having a baby with the disease. This does NOT mean one out of four of their kids will have it! The "dice is rolled" each time they have a kid. The chance of 3 of their kids having it are 1/64 because you do 1/4 times 1/4 times 1/4 since they're all independent events.

 

Sum Rule - the probability that one of two or more mutually exclusive outcomes will occur is the sum of the probabilities of the individual outcomes.

 

EXAMPLE: If you want to find out the probability of the off spring being X OR Y, you take the probability of X and add it to the probability of Y.

 

 

 

 

 

 

Interesteresterting Thingys:

 

PubMed Article: Genotype-phenotype associations and human eye color

http://www.ncbi.nlm.nih.gov/pubmed/20944644

 

Normally when talking about traits and stuff, we only talk about two alleles. However, some genes are affected by many genes, such as human eye color. It's controlled by about 16 different genes, but the main contributions come from two adjacent genes located on chromosome 15. This is why eyes vary so widely in color. Some people have two phenotypes, like my godmother who has one blue eye and one green eye. Other people have no pigmentation at all (albinos). How does this relate to the chapter? Along with epistasis, this is an example of incomplete dominance, which is explained in section 16.5.

 

This is a really freaking close up picture of the X and Y chromosomes. As you can clearly see, the X chromosome is a LOT bigger than the Y chromosome. This explains why females are superior to males. but on a more serious note, you know males have both X and Y sperm. The X sperm has 2.9% more DNA than the Y sperm. While the chromosomes are different in size, the X and Y sperm are the same size, so it's impossible to tell the difference by looking at size and shape. (Information and picture taken from this website: http://www.in-gender.com/cs/blogs/gender_selection_news/archive/2006/01.aspx)

 

 

Video Presentation: Codominance

http://www.youtube.com/watch?v=EhaobvCp_Oc

 

This video explains codominance. That's when you have a heterozygote and both alleles can be expressed simultaneously. An example the video gives is sickle-cell anemia. If a homozygous non affected and homozygous affected person have kids, all of them will be heterozygous. It's not like it's dominant so all red blood cells are sickled, and it's not even recessive so none are sickled but they're carriers. The phenotype of the children will be half of their blood cells are normal and half of them are sickled. Blood type is also codominant.