# Beyond Mendel’s Laws of Inheritance

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Beyond Mendel’s Laws of Inheritance

Rules of Probability Addition Rule- probability that any one of two or more mutually exclusive events will occur is calculated by adding the individual probabilities. Example- the chance of getting heads or tails on a coin toss heads- ½ tails – ½ ½ +1/2 = 1

Rules of Probability Multiplication- probability that two or more independent events will occur together Ex- What is the chance that two coins tossed together will both land heads up? probability of 1 head- ½ probability of 2 heads ½ x ½ = 1/4

Probability Problems Chance of rolling a three on a die? 1/6
Chance of getting any other number other than three? 5/6 Chance of rolling a 1 and then a 2 and then 4? 1/6 x 1/6 x 1/6 = 1/216 Parents are PP and pp- chance that offspring will be Pp? 1 x 1 = 1 Parents are PpYy x Ppyy chance that offspring will be ppYy? Pp= ½ x ½ Yy= ½ x 1 total=1/8

Incomplete dominance Heterozygote shows an intermediate, blended phenotype example: RR = red flowers rr = white flowers Rr = pink flowers make 50% less color RR WW RW RR RW WW

Incomplete dominance P F1 100% 1:2:1 F2 X true-breeding red flowers
white flowers 100% 100% pink flowers F1 generation (hybrids) self-pollinate 25% white F2 generation red 1:2:1 50% pink

Codominance There are two or more alleles that are dominant in a phenotype. Both alleles are expressed in heterozygous condition. Ex: Red – RR, white – WW, red and white - RW

Multiple Alleles 2 alleles affect the phenotype equally & separately
not blended phenotype human ABO blood groups 3 alleles IA, IB, i IA & IB alleles are co-dominant glycoprotein antigens on RBC IAIB = both antigens are produced i allele recessive to both

Polygenic inheritance
Some phenotypes determined by additive effects of 2 or more genes on a single characteristic phenotypes on a continuum human traits skin color height weight intelligence behaviors

Sex linked traits 1910 | 1933 Genes are on sex chromosomes
as opposed to autosomal chromosomes first discovered by T.H. Morgan at Columbia U. Drosophila breeding good genetic subject prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male

Hemophilia XHXh XHY Hh x HH XH XHXh XH Y Xh XHXH XHXH XHY XHY XH Xh XH
sex-linked recessive Hemophilia XHXh XHY Hh x HH XH XHXh XH Y male / sperm Xh XHXH XHXH XHY XHY XH Xh female / eggs XH XHY XHXh XHXh XhY XhY Y carrier disease

Pedigree Analysis Used to predict inheritance based on past phenotypes

Autosomal Recessive Inheritance-
cystic fibrosis Generation skipped

Autosomal Dominant Inheritance-Huntington Disease
Generations are not skipped

Human disorders The family pedigree
Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell Dominant disorders: -Huntington’s

Practice Question Sickle cell anemia is caused by the mutation of a single base pair in the gene for hemoglobin. This mutation results in hemoglobin molecules that form long chains in low-oxygen settings and stretch the blood cells into their characteristic sickled shape. What would the result be if a different base pair in the sickle cell hemoglobin gene were to become mutated? A. The effects of sickle cell would be increased B. The effects of sickle cell would be reduced C. The new mutation would increase resistance to malaria D. It is impossible to predict what the effect would be without knowing more about the mutation

Practice Problem In the cross AaBBcc x aaBbCc, what is the probability of an offspring that is aaBbcc? A 1/2 B 1/4 C 1/8 D 1/16

Practice Problem What type of inheritance pattern is illustrated above?

Genetics of Sex X Y X XX XY X XX XY
In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes develop as a female: XX gene redundancy, like autosomal chromosomes an X & Y chromosome develop as a male: XY no redundancy X Y X XX XY X XX XY 50% female : 50% male

true-breeding red-eye female true-breeding white-eye male X P Huh! Sex matters?! 100% red eye offspring F1 generation (hybrids) 100% red-eye female 50% red-eye male 50% white eye male F2 generation

Genes on sex chromosomes
Y chromosome few genes other than SRY sex-determining region master regulator for maleness turns on genes for production of male hormones X chromosome traits other than sex determination mutations: (all are recessive) hemophilia Duchenne muscular dystrophy color-blindness Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. The disease is characterized by a progressive weakening of the muscles and loss of coordination.

X-inactivation Female mammals inherit 2 X chromosomes XH XHXh Xh
one X becomes inactivated during embryonic development condenses into compact object = Barr body which X becomes Barr body is random patchwork trait = “mosaic” patches of black XH XHXh Xh tricolor cats can only be female patches of orange

Sex Linked Traits Usually found on the X chromosome
Only if she is homozygous recessive, will she exhibit the trait Expressed more with males since they have only one X

Duchenne muscular dystrophy Hemophilia Colorblindness

Hemophilia in the Russian Family

Linked Genes Are physically close to each other and get inherited together even with crossing over The greater the distance between the genes, the greater the chance of separation during crossing over

Map Units Morgan and Sturtevant developed the genetic map that shows a list of ordered loci on a chromosome The distance between the genes is called a map unit A recombinant frequency of 1% = 1 map unit

Example If the black body gene (b) is 17% or 17 map units from the vestigal wing gene (vg) and the cinnabar eye gene (cn) is 9.75% or 9.75 map units from the b gene, what are the order of the genes on the chromosome?

Cytogenic Maps Locates genes with respect to chromosomal features as stained bands

Alternative Systems

Nondisjunction

Remember this Down Syndrome

Aneuploidy- too many or too few
Plants- are actually less serious Animals (one too many or too few chromosomes), are very serious or fatal due to gene imbalances

XO Turner Syndrome- Monosomy

XXY- Klinefelter Syndrome

XYY- Jacob’s Syndrome Tall, thin, larger build More prone to acne
Increased testosterone levels

Genomic Imprinting Some traits vary depending on which parent passes it on The section of DNA has a methyl group (-CH3) added on Certain cancers can be stimulated this way Fragile X syndrome can come from father

Mitochondrial Defects
These defects originate in the mitochondrial DNA. Some examples are: Some forms of Diabetes Some forms of Deafness Multiple sclerosis Some forms of epilepsy

Environmental effects
Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences Color of Hydrangea flowers is influenced by soil pH

Human Genetic Diseases to Know
Huntington’s (AD) Colorblindness (SR) Hemophilia (SR) Cystic Fibrosis (AR) Sickle cell anemia (AR) PKU (AR) Tay Sachs (AR)