Download Mendelian genetics complete

Name _____________________________________________________________________
Test Date _Fri, 2/3_____
UNIT 8 - INTRODUCTION TO GENETICS
Although the resemblance between generations of organisms had been noted for thousands of years, it wasn’t until the 1800s that
scientific studies were carried out to develop an explanation for this. Today we know that we resemble our parents because of
_heredity_____, which is the set of characteristics we receive from _our parents_______. The study of heredity is known as
_genetics______.
I. SEXUAL REPRODUCTION & MEIOSIS
(pp. 275-278)
In sexual reproduction, an egg and sperm cell fuse together to create a fertilized egg or _zygote_.
A. Chromosome Number
1. Somatic Cells - _Body___ cells
 Human somatic cells contain _46___ chromosomes
 _Diploid____ or _2n___ meaning they contain a _double___ set of chromosomes, half ( _23__ ) from _Dad___ and
half from _Mom____.
 “Matching” chromosomes are known as _homologous pairs____. A homologous pair is made up of a copy of a
chromosome from each parent, with the same _genes____.
2. Gametes - _Egg___ and _Sperm_____ cells
 Human gametes contain _23__ chromosomes.
 _Haploid____ or _n___ meaning there is _one__ set of instructions for each _gene__.
 When gametes fuse together in _fertilization______, the _zygote____ produced is _diploid____ and has _46____
chromosomes.
B. Meiosis
 Special type of cell division that only occurs in specialized germ cells in _ovaries____ of females and _testes____ of males.
 In meiosis, DNA is replicated once but cell divides _2x____, resulting in _4_____ cells with _1/2___ the original chromosome
number.
 In females, process is known as _oogenesis______________.
 In males, process is known as _spermatogenesis_______.
 In both males & females, prior to meiosis I, DNA is replicated during _S__ of _interphase______.
Draw a chromosome before replication
Draw a chromosome after replication
Draw a chromosome before replication.
Draw a chromosome before replication.

Meiosis occurs in two stages:
1. Meiosis I
 Prophase I
 Unlike prophase of mitosis, _homologous pairs_____ come together to form a _tetrad____.
 Tetrads are held together at the _centromere______.
 Crossing Over
 Exchange of genetic information between a _sister chromatid___ with its _non-sister homologue____.
 Occurs very frequently
 Allows for _genetic variation__________.



Draw a homologous pair of
Draw a homologus pair
Metaphase I
chromosomes
of chromosomes after
 _Tetrads______ align in equator of cell
crossing over
 Each homologue consists of _two sister chromatids_________.
Anaphase I
 _Homologous pairs______ are pulled apart
 _Centromeres____, _sister chromatids______ still intact
Telophase I
 Two cells are formed, each with _23___ chromosomes
 Each chromosome still composed of two _sister chromatids____
 Two cells produced at the end of meiosis I are _haploid (n)__ because _there are no homologous pairs
present___
2. Meiosis II




Continues with the two cells formed moving directly into prophase II without any further _replication_____ of DNA.
In anaphase II, _sister chromatids_______ are pulled apart.
Two new cells are formed from each of the two cells formed in meiosis I, resulting in a total of _4___ new cells, each with
_1/2____the original number of chromosomes.
Cells produced are called _gametes_______.
3. Oogenesis vs. Spermatogenesis
II. HISTORY OF GENETICS
(pp. 263-270)
A. Gregor Mendel
 Known as the “Father of _Genetics_____”
 Famous for his experiments with _pea__ plants.
 Used true-breeding pea plants, which means _pureline, purebred____; characteristics always show. Known as the
_P____ generation.
 Studied seven _traits__, including plant height, seed color, flower color, etc.
o A trait is an _inherited characteristic______.
 Pea plants cross-pollinate, meaning pollen from one plant fertilizes an egg from another, but they can also selfpollinate, meaning pollen can fertilize egg from _same____ plant. Mendel controlled the fertilization process of the pea
plants by preventing _self-pollination____ and controlling _cross-pollination_____.
B. Mendel’s Results
 P generation – Crossed _true-breeding___ plants with one trait with _true-breeding__ plants with the other. For
example, __TT (tall) x tt (short)________
 F1 generation – Offspring produced from _P x P_________. In F1, one trait _disappeared____. For example, tall
plants X short plants = _all tall plants_________.
 F2 generation – Offspring produced from _F1 x F1________. In F2, trait that disappeared in F1 reappeared in __1/4___
of the offspring; the other ¾ showed _dominant trait_____.
C. Mendel’s Principles – After analyzing his results carefully, Mendel formed conclusions that increased understanding of inheritance
and opened the door for the study of genetics.
 Individual units called _genes____ determine inheritable characteristics. A gene is a portion of _DNA___ that codes
for a specific _trait____.
 For each gene, an organism inherits two alleles, one from each _parent_____. Alleles are different forms or
_versions_____ of a _gene____. For any given trait,
o If an organism is _homozygous___, its alleles are the same and the trait will be expressed.
o If the alleles differ, the organism is said to be _heterozygous______ for that trait and only one allele
will be expressed. The expressed allele is the _dominant_____ allele, designated by an _upper___-case letter.
The allele that is not expressed in a heterozygous trait is _recessive_______, designated by a _lower__-case
letter. A recessive allele is only expressed when an organism is _homozygous____.
 Principle of Segregation - In meiosis, the two alleles for a trait segregate (_separate________). Each egg or sperm cell
receives a copy of one of the two alleles present in the somatic cells of the organism. There is a _50%____ chance that a
copy of that allele will end up in the gamete produced.
 Principle of Independent Assortment – The way one pair of alleles segregates has no influence on any other pair of
alleles.
D. Genetics Terminology
 Phenotype - _Physical__ description of trait; for example, _tall, short_______
 Genotype – _Genetic make-up____ of an organism; for example, _TT, Tt, heterozygous, etc_____

If round pea seeds are dominant to wrinkled pea seeds, round is designated _R_ and wrinkled is designated _r___.
o Homozygous dominant
 Genotype = _RR_____________________
 Phenotype = _round__________________
o Heterozygous
 Genotype = _Rr______________________
 Phenotype = _round__________________
o Homozygous recessive
 Genotype = _rr_______________________
 Phenotype = _wrinkled________________
III. ANALYZING INHERITANCE
(pp.135-137)
A. Probability
Due to the Law of _Segregation_______, if you know the genotype of the parents, you can predict the likelihood of a trait
occurring in the offspring. Probability can be written 3 ways; for example, the probability of a coin coming up heads after being
flipped is (fraction) _1/2_, (ratio) _1 : 2____, or (percent) _50%___.
B. Punnett Squares
A Punnett square is a tool used to predict the possible outcomes of _meiosis___ and _fertilization___; in other words, a Punnett
square is used to determine the probability of certain traits appearing in offspring.
IV. PUNNETT PRACTICE
**Please note: To earn full credit, you must include a key and cross with each problem!**
A. Construct a Punnett square to determine the probability of white flowers if a heterozygous purple (Pp) flower is
crossed with a homozygous white (pp) flower.
Key: _________________________________________
Cross: ________________________________________
Probability of white flowers = ___________________
B. Construct a Punnett square to determine the probability of short pea plants if a homozygous tall (TT) plant is crossed
with a heterozygous tall (Tt) plant.
Key: _________________________________________
Cross: ________________________________________
Probability of short pea plants = _________________
Probability of tall pea plants = ___________________
C. If round peas are dominant over wrinkled peas, make a Punnett square to determine the genotype and phenotype ratios of the
offspring if a heterozygous plant is crossed with a homozygous recessive plant.
Key: _________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
D. Use a Punnett square to determine the genotype and phenotype ratios of the offspring from a cross between a homozygous
dominant yellow pea pea plant and a homozygous recessive green pea pea plant.
Key: _________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
V. DIHYBRID CROSSES
(pp. 270-271)
The Punnett squares we have been doing are known as _monohybrid_____ crosses, meaning that only one trait has been
considered at a time. In a dihybrid cross, _2___ different _genes____ on 2 different _chromosomes______ are analyzed.
A. Peas homozygous for round shape and heterozygous for color are crossed with yellow peas heterozygous for shape
Key: R = round, r = wrinkled; Y = yellow, y = green
Cross: ________________________________________
Genotype ratio: _______________________________________________________________________________
Phenotype ratio: ______________________________________________________________________________
B. Key: G = gray body, g = black body; R = red eyes, r = black eyes
Cross: GGRr X Ggrr

What are the phenotypes of the parent fruit flies? _________________________________
Cross: __________________________________________
Genotype ratio: _________________________________________________________________________
_____________________________________________________________________________________
Phenotype ratio: ________________________________________________________________________
______________________________________________________________________________________
VI. A CLOSER LOOK AT HEREDITY
(pp. 272, 273)
A. Incomplete Dominance – Neither allele has “complete” dominance over the other - heterozygous phenotype is a
_blend of two homozygous phenotypes_________

For example, in snapdragons, ____________________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
B. Codominance – Both alleles _share_____ dominance and are always _show_____ if present.

For example, ________________________________________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
Cross: ________________________________________
Genotype ratio: _________________________________________
Phenotype ratio: ________________________________________
C. Polygenic Traits – “_Many genes_______” Many traits are controlled by more than one gene. Examples include _skin tone,
height_________________________
D. Multiple Alleles – Many genes have more than _2___ alleles, although an individual only has _2___ alleles for the gene. An
example is _human blood types___. There are _3__ possible alleles for this gene.
Suggested Study Questions
p. 283 (1-5, 11, 12, 17-20)