GENETICSCHAPTERS 9 & 12

I. The History of GeneticsGenetics = field of biology devoted to understanding how traits are passed from parent to offspring

Heredity

= the transmission of characteristics from parent to offspring

Mendel’s Experiments

Gregor Mendel – the father of genetics

Mendel’s Experiments Continued…Mendel’s Experiments (1856 – 1863)

Observed characteristics of pea plants (7) over a long period of time (28,000 plants over 8 years!)

Initial observation = when he planted purple pea plants some white pea plants grew

Mendel’s Methods:

1. grew pure purple and pure white plants for several generations (P1 generation = parent generation)

Mendel’s Methods continued…2. cut off anthers from purple plants (male part)

Mendel’s Methods continued…3. cross-pollinated purple and white pure strains

4. F1 generation grew (first filial generation) and he counted the number of purple and white plants

5. all F1 plants were purple

Mendel’s Methods continued…

6. allowed self-pollination of F1 generation, producing the F2 generation

7. 75% were purple25% were white

Mendel’s Conclusions:

1. something within the pea plants controlled the characteristics he

observed = “factors”

2. each trait was inherited by a separate “factor”, since he observed 2

forms of each trait he hypothesized each characteristic came in pairs

Mendel’s traits were carried by “factors” in each plant

Each trait had 2 possible “factors” (pairs)

Mendel’s Conclusions continued…3. since 1 trait completely disappeared in the F1 generation, but appeared in a 3:1 ratio in the F2 generation, he

hypothesized that one of these “factors” was dominant and one was

recessive = Rule of Dominance

Laws that Evolved from Mendel’s Work

The Law of Segregation = The two alleles (“factors”) for each trait must separate during gamete formation (meiosis). There will only be one allele for a trait in each gamete.

Laws the Evolved from Mendel

The Law of Independent Assortment = the alleles for each trait must behave independently of alleles for other traits during gamete formation. We have no way of determining which allele will go into which gamete, it is random!

Mendel’s Work led to…

The Scientific Field of Molecular Genetics

= study of structure and function of chromosomes and genes

**Mendel’s “factors” = genes

The Discovery of

DNAThe m

apping of the human

genome project

Genetic

Engineerin

g

MOLECULAR GENETICS

Mendel did not know the structure of DNA or that there were genes during his research

Gene = segment of DNA on a CH that controls a trait

Chromosomes (CH’s) occur in pairs = there are 2 genes responsible for each trait 1 from mom 1 from dad

Molecular Genetics continued…

Allele = each different form of a gene, during meiosis each gamete gets one form of this gene

Rr R = dominant alleler = recessive allele

Example of Alleles

How to follow allele inheritance= Genetic Crosses

Genetic Crosses Objectives

1. Phenotype of the parents & offspring

= how a trait shows up physically in the organism

Genetic Crosses Objectives

2. Genotype= the alleles (genes) that the organism has inherited

homozygous (the same alleles) recessive rr dominant RR

heterozygous (different alleles) Rr

Genotype

Type of Crosses and Inheritance

1. Monohybrid cross = following inheritance of 1 trait from generation to generation

Types of Crosses continued…2. Dihybrid = following inheritance of 2 traits from generation to generation

Crosses

3. complete dominance = dominant phenotype always masks the recessive phenotype when the dominant allele is present

Crosses

4. incomplete dominance = both phenotypes mix together in a heterozygous individual

Crosses

5. codominance = both phenotypes show up equally in a heterozygous individual

B = Black catW = White cat

In order to set up a cross, you use

1. Monohybrid Cross Punnett Square1 trait!

2. Dihybrid Cross Punnett Square2 traits!

What if you don’t know the genotype of an organism…only the phenotype

You Perform A Test Cross

= used if the individual of interest shows the dominant phenotype (physical characteristic) but you don’t know if it is heterozygous or homozygous dominant genotypically (what alleles the individual actually inherited)

A TEST CROSS

= cross that unknown individual with a homozygous recessive and look at their offspring

Test Cross Results

Look at offpring of test individual and the homozygous recessive parent

All dominant offspring = test individual is homozygous dominant (PP)

Any recessive offspring at all = test individual is heterozygous (Pp)

Another Test Cross Example

Probability

Probability = likelihood that a specific event will occur, expressed as a decimal, percentage or fraction

# times an event is expected to happen

# of opportunities for an event to happen

Chapter 12 – Human Inheritance

Types of Genetic Traits/Disorders & how they are inherited:

1. Single-allele traits = controlled by 1 allele of a gene (200)

i.e. Huntington’s Disease (HD), Cystic fibrosis, sickle cell anemia

Human Inheritance Types…

2. Multiple-allele traits = controlled by 3+ alleles of the same gene that code for a single trait i.e blood types

AA = buds SS = spikes AS = spikes & buds NN= none AN = buds SN = spikes

Universal Acceptor

UniversalDonor

Human Inheritance Types…

3. Polygenic Traits= a trait controlled by several genes i.e. skin color, eye color, human height

Human Inheritance Types…

5. Sex-Linked/Sex-Influenced traits = presence of male or female chromosome and sex hormones

Sex-Linked – the gene for trait is on the X or Yv.

Sex-Influenced – the gene isn’t on the X or Y, but the expression (phenotype) is affected by the hormones made by being XX (female) or XY (male)

Sex-Linked

Examples of Sex-Linked Traitsi.e. hemophilia, colorblindness, muscular dystrophy, baldness

Sex-Linkage

= presence of a gene on a sex chromosome (X or Y)

1) X-linked – genes found on the X chromosome and inherited when the individual receives an X during fertilization Most common type

Most X-linked diseases/traits are recessive (need 2 copies of allele to show disease/trait physically)

X-linked continued…

Father to son transmission is IMPOSSIBLE!

You can be a “carrier” and not have the disease

More common in males! (they only need 1 copy!)

Examples: muscular dystrophy, hemophilia, color-blindness, fragile X-syndrome, protanopia, Aicardi syndrome

 

MaleThe "a"

recessive allele will beexpressed in

his phenotype   

FemaleThe "a"

recessive allele will notbe expressed

in her phenotype

Y-Linked Traits

= genes found on the Y chromosome and inherited when the individual receives a Y (and become a male) during fertilization

Only males can have these diseases/traits

Very rare

Examples: Klinefelter syndrome (XXY), Jacobs Syndrome (XYY)

What does it mean to “carry” a trait?

Carrier= when an organism has 1 copy of an allele that causes a recessive disorder but does not present that disorder/trait physically

XdX = carrier, femaleXdXd = has disorder, femaleXdY = has disorder, maleXY = doesn’t have disorder and is

not a carrier!, male

= A PEDIGREE

What is a PEDIGREE??

= family record that shows how a trait is inherited

over several generations ~ is actual inheritance

and not “possible”

Shows:1.Sex of

individuals2.“marriages”3.Number of

offspring4.Type of trait

a. Single-allele

b. Sex-linked5.Genotypes6.relatedness

How to Read a Pedigree