M S . N A P O L I T A N O & M R S . H A A SC P B I O L O G Y

GENETICS

INTRODUCTION TO GENETICS

• Every living thing inherits characteristics from its parent(s)

• Genetics – the scientific study of heredity

GREGOR MENDEL & HIS PEA PLANTS

• Studied genetics of garden peas

• Started with true-breeding plants• If allowed to self-pollinate, would create

identical offspring• One only tall, one only short, one green, one

yellow, etc. (called traits)

• Self-pollinated the plants by hand• Controlled breeding• Called “cross-pollination”

GENES & DOMINANCE

• Traits – specific characteristics that vary from one individual to another• Freckles, eye color, right vs. left handed, etc.

• Original cross – P generation• Mendel = true breeding plants

• Offspring = F1 generation

• Offspring of F1 = F2, etc.

• Hybrids – parents have different traits

MENDEL’S PEA PLANTS

MENDEL’S CONCLUSIONS

1. Inheritance determined by factors that are passed down from one generation to the next• Genes – chemical factors that determine traits• Alleles – different forms of a gene• Ex: brown vs. blue eyes, curly vs. straight hair, etc.

2. Principle of dominance – some alleles are dominant and others are recessive • Dominant (T) will always be expressed • Recessive (t) only expressed if dominant allele is absent

MENDEL’S SECOND CROSS

• Mendel not satisfied – what happened to recessive alleles?

• Crossed F1 generations with themselves• Made F2 generations

• Results – traits from recessive alleles came back in ~¼ of plants

SEGREGATION

• Segregation – separation of alleles• Happens during meiosis during formation of

gametes

Mendel’s conclusions:• When F1 plants produce gametes, alleles separate

so that each gamete carries only a single copy of the gene• F1 produces two types of gametes – one tall allele

& one short allele

EXAMPLE OF SEGREGATION

GENETICS & PROBABILITY

• Probability – the likelihood that a particular event will occur

• Probability is used to predict the outcomes of genetic crosses

• Punnett square – used to predict genetic variations• Homozygous – 2 identical alleles for a particular trait • TT or tt• Aka “true breeding”

• Heterozygous – 2 different alleles for a particular trait • Tt• Aka “hybrid”

GENOTYPE & PHENOTYPE

• Phenotype – physical characteristics• Ex: Plant phenotype is tall or short

• Genotype – genetic makeup• Ex: Plant genotype is TT, Tt, or tt

PUNNETT SQUARE – PEA SEED COLOR

PUNNETT SQUARE – PEA SEED COLOR

1. What is the genotype of both parents?2. What is the phenotype of both parents?3. Are the parents homozygous or heterozygous?

PROBABILITY & SEGREGATION

• In previous Punnett square, ¼ seeds are green and ¾ seeds are yellow • 3 dominant, 1 recessive• Ratio = 3:1

• In previous Punnett square, ¼ seeds are YY, ¼ seeds are yy, and 2/4 (1/2) seeds are Yy• Ratio = 1:2:1

PROBABILITIES & AVERAGES

• Need a large sample size!• Probability cannot predict outcome of a single

event• Ex: In theory, if you flipped a coin twice, you

would get 1 heads and 1 tails – but won’t always happen• The larger the number of offspring, the closer the

resulting numbers will get to expected values

INDEPENDENT ASSORTMENT

• Does segregation of one pair of alleles affect the segregation of another pair of alleles?

• Mendel – crossed true-breeding, round, yellow peas (RRYY) with true-breeding, wrinkled, green peas (rryy)

• Offspring: all round, yellow peas

• Therefore: Yellow & round dominant to wrinkled & green

INDEPENDENT ASSORTMENT

• Mendel – now crossed new plants (RRYY x rryy)• 556 total seeds produced• 315 seeds round, yellow• 32 seeds wrinkled, green• 209 seeds had combinations not found in either parent

• Independent assortment – alleles segregate independently• In this example, color segregates independent of shape• In other words – the inheritance of one gene does not

affect the inheritance of the other

• Mendels results close to 9:3:3:1 ratio

MENDEL’S SECOND CROSS

GENETIC DISEASES

• Tay Sachs• Recessive • Progressively destroys nerve cells

• Sickle Cell Anemia• Recessive• Affects hemoglobin shape in red blood cells• Cannot get enough oxygen to cells

• Huntington’s Disease• Dominant (ah!)• Neurodegerative• Cognitive decline• Symptoms do not arise until ~35-44 years old

EXCEPTIONS

• Genetics is very complicated in many organisms! • Many genes have more than 2 alleles

• Some alleles are neither dominant nor recessive

• Examples: • Incomplete dominance• Codominance

INCOMPLETE DOMINANCE

• Example: A white & red flower cross, making pink• One allele is not completely dominant over

another• Traits “blend”

Parents:• 1 WW (white)• 1 RR (red)

Offspring• 4 RW (pink)

CODOMINANCE

• Both alleles contribute to phenotype• Both traits fully expressed• Ex: In chickens, black feathers is codominant with

white feathers

Parents• 1 CBCB

• 1 CWCW

Offspring• 4 CBCW

MULTIPLE ALLELES

• Genes with more than 2 alleles• More than 2 alleles possible in a population (Not

per organism! Remember – 1 from mom, 1 from dad)• Ex: Rabbit coat color

SEX-LINKED GENES

• Sex-linked genes - genes located on sex chromosomes

• Mostly found on the X chromosome• Much bigger in size• Over 100 sex-linked diseases on X chromosome

• Males will express X-linked alleles because they have only 1 X chromosome!• Doesn’t matter if dominant or recessive

• In Punnett square, only the X chromosome gets the gene!• Ex: Parents will be XBXb and XbY

SEX-LINKED PUNNETT SQUARE

SEX-LINKED DISEASES

• Colorblindness• Inability to distinguish certain colors• Red-green colorblindness = 1 in 10 males in US• Only 1 in 100 females are colorblind (why?)

• Hemophilia• Blood-clotting protein missing, can bleed to death easily• 1 in 10,000 males have hemophilia

• Duchenne Muscular Dystrophy• Progressive weakening & loss of skeletal muscle• 1 in 3,000 males

BLOOD TYPING

• Use ABO and Rh blood groups• Rh = either + or –, + is dominant

• 3 alleles: IA, IB, & i• IA and IB are codominant, and both dominant to

recessive i• Possible blood types:• IA IA – Type A• IA i – Type A• IB IB – Type B• IB i – Type B• IA IB – Type AB• i i – Type O

BLOOD TYPE PUNNETT SQUARES

POLYGENIC TRAITS

• Polygenic traits - traits controlled by 2+ genes• Ex: Human skin color (4 genes)

HUMAN CHROMOSOMES

• Recall: Humans have 46 chromosomes

• Karyotype – picture of human chromosomes lined up in order

• 2 chromosomes = sex chromosomes• Determine gender• Females = XX• Males = XY

• Other 44 chromosomes = autosomes

HUMAN KARYOTYPE

THE “MAGIC” MOMENT

• Males & females born in ~50:50 ratio• All human egg cells carry a single X chromosome,

but only half of all human sperm cells carry a single X chromosome. The others carry a single Y chromosome.• Females = (23,X)• Males = (23,Y)

X

X Y

X+

+

=

=

Female

Male

PEDIGREE CHARTS

• Show relationships within a family

PEDIGREE EXAMPLE