Genetics: Day 4

Theoretical Genetics

• In 1865 Austrian monk Gregor Mendel published the results of his experiments on pea plants; why he is often referred to as Father of Genetics

Vocab Review• Genotype – the genetic make-up of an organism

represented by two letters (Bb,bb,BB)• Phenotype – organism’s appearance or observable

traits based on the genotype• Dominant allele – the allele expressed in the

phenotype (BB or Bb, either way is expressed)• Recessive allele – allele that effects the phenotype

only when in homozygous (bb)• Codominant alleles – pairs of alleles that both affect

the phenotype when present in a heterozygote (e.g., curly hair + straight hair = wavy hair)

• Homozygous – having two identical alleles of a gene (BB or bb)

• Heterozygous – having two different alleles of a gene (Bb)

• Carrier – individuals who carry a recessive allele for a disease or defect without manifesting the condition

• Test cross – breeding an organism of unknown genotype with a recessive homozygote (done in order to reveal that organisms genotype since it may be unknown if it is BB or Bb)

Monohybrid cross with Punnett grid

Genotype:

¼ homo dominant ½ hetero ¼ homo recessive ratio 1:2:1

Phenotype:

¾ purple ¼ white ratio 3:1

Mendel’s Law of Segregation

• After completing his experiments on pea plants Mendel came up with a law that explains the behavior of alleles:The law states that the two alleles for a certain characteristic/trait segregate during gamete formation and end up in different gametes.

Multiple alleles• Blood types in humans determined by 3 alleles

which can produce 6 different genotypes and 4 phenotypes including A, B, AB, O

• IA= allele for type A blood• IB= allele for type B blood• i = allele for type O blood

(the letters refer to a carbohydrate found on the surface of red blood cells; type A has A carb, type B has B carb, type AB has both carbs and type O has neither carb)

Note: You can also further specify blood types based on the presence or absence of the Rh protein (+ with protein and - without)

Sex-Linked Genes• The 23rd pair of chromosomes in humans called

sex chromosomes (other 22 are autosomal)• Females are XX and males are XY• Males determine the gender at conception

since through Meiosis either an X or a Y chromosome was passed on

• Sex-linked genes are located on a sex chromosome but typically it is referring to a gene on the X chromosome (the result of this is often detrimental to males since they do not have another X to counteract)

Remember, X and Y chromosomes are very different in size and therefore contain different numbers of genes

Examples of Sex-Linked ConditionsI. Color blindness• Xb= recessive allele for color blindness• XB= ability to see color• For a man to be colorblind he must inherit

the recessive allele (Xb) from his mother (who must either be a carrier or colorblind)

• For a woman to be colorblind she must inherit both recessive alleles, one from her father (who is colorblind) and one from her mother (who is a carrier)

Examples of Sex-Linked Conditions

II. Hemophilia• Xh= allele for hemophilia• XH= allele for the ability

to clot blood• Works the same as color

blindness and therefore most humans with hemophilia are male

Since females have two X chromosomes (which are much larger than the Y chromosome), do females code for 2x the proteins of males?

The answer is no. One X chromosome in each cell of a female becomes almost completely inactivated during embryonic development. The inactive X chromosome condenses into a Barr body. Each cell randomly and independently “chooses” which X chromosome to inactivate.

Interesting Note on X Inactivation

• Tortoiseshell cats (which must be female) caused by the inheritance of both the black fur and orange fur alleles; these alleles are located on X chromosome