1. 2013 Winter Webinar Series: Breeding Better Sheep & Goats
Genetics 101
SUSAN SCHOENIAN
Sheep & Goat Specialist
University of Maryland Extension
sschoen@umd.edu - www.sheepandgoat.com

2. DNA, chromosomes,
genes, and alleles

3. DNA (deoxyribonucleic acid)
• The genetic material that
controls how an animal
looks and performs.
• Exists as two long strands
spiraled into a double
helix.
• Is organized into
chromosomes.

4. Chromosomes
• Found in pairs in the
nucleus of every cell
of the body.
• There are two kinds
of chromosomes.
1. Autosomes
2. Sex chromosomes

5. Species differ in the number of
chromosomes they have.
n=60
n=54 30 pairs
27 pairs

6. AUTOSOMES SEX CHROMOSOMES
• Comprise all but one pair • Every animal has one pair
of chromosomes. of sex chromosomes.
• (Mostly) control features • There are two types of sex
the same in male and chromosomes denoted by
females.
the letters X and Y.
• Females have two of the
same kind of sex
chromosomes (XX).
• Males have two distinct sex
chromosomes (XY).

7. Sex determination
• The sex of the
offspring is
determined by the
sex chromosomes.
• Males contribute
either an X or Y
chromosome, thus
determine the sex
of the offspring.
• Diet may also affect
sex ratio.

8. Genes
• A unit of inheritance
composed of a segment
of DNA.
• Also in pairs.
• Encodes the amino acid
sequence of a protein.
• Physical location of
gene on DNA molecule
is called a locus (loci).

9. Alleles (usually denoted by letters)
• An alternative form of a
gene (one of the pair)
that is located at a
specific position of a
specific chromosome.
• Organisms have two
alleles for each trait or
the expression of a trait
may be affected by
multiple alleles.

10. HOMOZYGOUS HETEROZYGOUS
• When the two alleles are • When the two alleles are
the same. different.

11. RECESSIVE (lower case letter)
• Allele that causes a
DOMINANT (Capital letter) phenotypic trait that is only
• One allele masks the seen in the homozygous
expression of another allele genotype.
at the same loci.
• Overrides the traits of a A a
recessive allele in a
heterozygous pairing. AA
A Aa
a Aa aa

12. Dominant traits in sheep and goats
• Hairy fleece
• Red, tan hair sheep
• White wool
(most breeds)
• White goats
• Red Boers
• Polled
• Brown eyes
• Wattles

13. Recessive traits in sheep and goats
• Woolly fleece
• Colored fleece
(most breeds)
• Horns
• Blue eyes
• Myotonia
• Colored goats
• Black Boers
• Genetic defects
– Spider lamb disease
– Cryptorchidism
– Entropion

14. Incomplete or partial dominance
• When the dominant
allele is only partially
dominant.
• Example: scurs

15. Sex-limited inheritance
• Sex-linked - on X or Y
chromosome
Example: hemophilia
• Sex-limited - all or none
expressed by sex
Example: milk production
• Sex-influenced - genotype
+ sex determines
phenotype
Examples: horns in most
sheep and beards in goats.

16. Epistasis
• When the action of one
gene depends upon
another.
Example: fleece color, skin
color, coat color
• Inheritance of fleece and
coat color can be
complicated and varies by
breed.

17. Genetic linkages or co-inheritance
• Traits that tend to be
inherited together as a
consequence of an
association between
their genes.
Examples: polledness
in goats and
hermaphrodism
(intersex in the
homozygous female).

18. Genetic correlations and responses
• Extent to which the
genotypic values for one
trait predict the genotypic
values for the second
trait.
• Can be a positive or
negative relationship.
-1 ↔ +1
• Correlation can also be
due to environmental
influences.

19. Positive correlations (0-1)
Birth weight Weaning weight
Weaning weight Post weaning weight
Post weaning weight Yearling weight
Rib eye area Percent retail cuts
Ovulation rate Litter size
Fecal egg count FAMACHA© score
Fecal consistency score Dag score
Frame size Carcass weight
Scrotal circumference Semen production
Ultrasound rib eye area Actual rib eye area

20. (-1 – 0) Negative correlations
Birth weight Lambing ease
Fleece weight Fiber diameter
Staple length Fiber diameter
Ovulation rate Embryo survival
Fecal egg count Packed cell volume

21. No correlation (zero)
Scrapie genotype Production traits
Resistance to a disease Resistance to another disease
Reproductive rate Growth and carcass traits
Reproductive rate Wool traits
Horn condition Productivity
Color Productivity
Genetic correlations tend to vary by breed and study.

22. Genetic defects
Simple recessive inheritance More complicated
• Spider lamb syndrome • Jaw defects
• Cryptorchidism • Entropion
• Myotonia • Hernias
• Hairy lamb syndrome • Teat defects
• Polledness in goats • Structural defects
• Fleece defects
• Rectal and vaginal prolapse
• Birth (congenital) defects

23. Inheritance
• Each sperm and egg
contains one
chromosome from each
pair of chromosomes of
the parent.
• Which chromosome
of a pair ends up in a
particular sperm or egg
is determined purely by
chance.

24. Inheritance
• There are 134,217,728
(227) possible
combinations of the 54
or 60 chromosomes in a
sperm or egg produced
by a male and female.
? • This results in
considerable variation
in the progeny from the
same mating.

25. Genotype vs. Phenotype

26. GENOTYPE (G) PHENOTYPE (P)
• The genetic make-up of a • The observable or
cell, organism, or animal. measurable traits of an
• Determines hereditary organism: what we can see
potential and limitations of or measure.
the individual.
• Determined by
genetics, plus
environmental influences
(E).
P=G+E

27. Environmental influences
• Diet
• Health
• Weather
• Age
• Type of birth and
rearing
• Age of dam
• Housing
• Season of birth

28. Which buck is better?

29. Two kinds of traits
1. Qualitative
2. Quantitative
(or polygenic)

30. QUANTITATIVE
QUALITATIVE • Continuous in
• Phenotype falls into expression.
specific category. • Considerable variation
• Usually affect by in phenotype.
one or few genes. • Under the influence
• Very little influence of many genes
of the environment. • Much more
environmental
influence

31. Qualitative traits
• Blood type
• Eye color
• Coat type
• Fleece or coat color
• Horns
• Wattles
• Beards
• Inherited defects
– Entropion
– Spider lamb disease
– Cryptorchidism
– Myotonia

32. Quantitative traits
• Reproductive rate
• Growth rate
• Milk production
• Fiber production
• Carcass characteristics
• Disease resistance
• Conformation
• Wool shedding
• Feed efficiency

33. Quantitative traits
• Are usually
the traits of
greatest
economic
importance.

34. Methods of genetic improvement
1. Crossbreeding
2. Selection

35. Next webinar: Jan 29, 7 pm EST
“Breeding systems” with Jeff Semler