Topic 3.4 InheritanceIB Biology

R. Price

v. 1 2015

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#1: Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed

Gregor Mendel• 1866: “Experiments in Plant

Hydridization”• Pea Plants: cross pollinate or self-

pollinate• Quantitative results• Large numbers of replicates• Seven different cross experiments• One father of genetics• Pioneer in research methods in Bio

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#2: Gametes are haploid so contain one allele of each gene

• Gametes = sex cells• Male gamete + female gamete =

zygote• Male gamete: generally smaller,

mobile• Contain one chromosome of each

type (haploid)• Therefore, only one allele of each

gene

#3: The two alleles of each gene separate into different haploid daughter nuclei during meiosis

• During meiosis a diploid nucleus divides twice to produce four haploid nuclei.• The diploid nucleus

contains two copies of each genes, but the haploid nuclei contain only one.

#4: Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles

ExampleTwo alleles possible: A or aZygote has 3 possibilities: AA, Aa, aa

More than one alleleABO blood type: IA IB i

Six possibilities: IA IA IB IB ii Iai IB i IA IB

#5: Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects

• The usual reason for dominance of one allele is that this allele codes for a protein that is active and carries out a function, whereas the recessive allele codes for a non-functional protein.

• Some genes have pairs of alleles where both have an effect when they are present together. They are called co-dominant alleles.

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#6: Many genetic diseases in humans are due to recessive alleles of autosomal genes

• Most genetic diseases are caused by a recessive allele of a gene• Most genetic diseases must have

*two* copies of an allele to develop• If someone has one dominant allele

and one allele for the genetic disease = “carrier”• If two carriers have a child, the

probability of their child to develop the disease = 25%

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#7: Some genetic diseases are sex-linked and some are due to dominant or co-dominant alleles

Sex-linked• Red-green color blindness• Hemophilia• Much more common in males than

females

#7: Some genetic diseases are sex-linked and some are due to dominant or co-dominant alleles

Dominant: Huntington’s Disease• Dominant allele• Located on chromosome 4. Gene

produces protein named huntingtin• Degenerative changes in the brain• Symptoms start between ages 30-50.• Severe changes in behavior, thinking,

emotions. Will eventually need full nursing care.

#7: Some genetic diseases are sex-linked and some are due to dominant or co-dominant alleles

Co-Dominant: Sickle Cell Anemia• Normal hemoglobin: HbA

• Sickle cell hemoglobin: HbS

• If HbA HbS then mild anemia, but increased resistance to malaria

#8: The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes

• Location on X-chromosome• Males: inherit X

from mother, can’t be “carriers”• Females: father

must have disorder, mother must pass on allele

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#9: Many genetic diseases have been identified in human but most are very rare

• More than 4,000 genetic diseases have been identified• Most caused by very rare recessive

alleles• Would have to inherit *two* very rare

recessive alleles

#10: Radiation and mutagenic chemicals increase the mutation rate and can cause genetic disease and cancer

Chemical Changes in DNA• Gamma rays, alpha

particles, UV, X-rays • Tobacco smoke, mustard

gas• Mutation of cell division

gene = cancer• Mutation of gamete DNA

= possible genetic disease for offspring

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Sources

Content Allott, Andrew, and David Mindorff. Biology: Course Companion. 2014 ed. Oxford: Oxford UP, 2014. Print. Oxford IB Diploma Programme.

Walpole, Brenda. Biology for the IB Diploma. 2nd ed. Cambridge: Cambridge UP, 2014. Print.

ImagesUnless otherwise noted, images are obtained from Pixabay (www.pixabay.com) and used under the CC0 Public Domain license.